This is how skills are supposed to work!

If you are in a situation where there is only one person doing something, and they are rolling a single skill check, then yes, this is how it's supposed to work. Giving help is a natural thing and should be used in situations like this. There is no reason to prevent it unless the task is clearly something that's not going to benefit from someone else giving you assistance. There are some things you can do to limit it.

It's also worth noting that helping can often save you some table time. As a AceCalhoun points out in the comments, in many cases what happens if you don't help is that everyone in the party tries their hand at the task. This behaves very much like advantage, but with a slightly lower overall modifier (because most likely you'll have one character who is good at a task and the rest that are lower). So Working together only raises the change of success slightly and consumes less table time in these cases.

• Be a bit more stringent about what you allow for assistance. Is coaching stealth really all that helpful? do you really want someone looking/talking over your shoulder while you're picking that lock? Evaluate situations where characters attempt to aid more carefully.

• Have more than one thing going on at once. If all the characters need to be stealthy, they can't be helping each other. And if you need two arcane characters working on the sigils on opposite sides of the room, maybe they have to choose which one gets help from the third (or don't have anyone to help at all).

• Make things take multiple rolls and limit helping on all of them. Maybe the first roll the wizard can be helped, but after that he's on the other side of the trap, or arm deep in the sigil or something to where additional assistance isn't going to help him.

• Figure out how to inflict disadvantage for the task. Maybe there are mitigating circumstances.

• Create a distraction. A rogue can't help the wizard if he's busy fighting baddies. Make some skill checks happen in an occupied room. Make completing the skill checks the win condition rather than defeating the enemies.

The basic crux of all of this is that helping is supposed to a mundane task that provides advantage. Yes, that's a huge deal, but it also doesn't stack with other things that give you advantage and it can be easily cancelled by disadvantage.

So get creative! Build some situations into your adventures that prevent your heroes from helping each other (or make the opportunity cost higher). But don't do it all the time, that might get tiresome. Adventurers like to help each other out, let them, but don't make it easy all the time.

Talk to the players.

The party isn't doing anything wrong per se. Proper use of choke-points is in fact good tactics, especially if they don't have a strong need to move in and surround the enemy. (If the party had a bunch of melee guys getting screwed over by the tactic, then it would stink.)

But the easiest thing might be to talk to the players. Acknowledge that they've done nothing actually wrong but bring up the points you have brought up here, that it's just not very much fun for you, and maybe for them.

Refuse to engage.

To quote the usual message: "If the party can do it, so can your monsters."

The players like to huddle up in a corridor and refuse to come out? Why would the monsters stand there in the open and try to break through instead of retreating and waiting for the PCs to move?

Encounters as laid out in the book are only the general suggestion of how something works, but unless your monsters are mindless or of merely animal intellect, they should be able to easily recognize the strategy and move to the sides, out of the line of fire of the "nukers" in back. Stalemates aren't fun, especially if there's also some incorporeals or patrolling monster squads hassling the back line (though that depends on the specific fight at hand). Keep in mind that fights are loud, and big evocations doubly so; you have every justification you need to bring random wandering patrols down on the party's back. Just be careful that this strategy makes sense in context; if the party gets jumped by patrols when they fight in hallways but doesn't when they go into the room, it's gonna feel unfair (because it is) and set up a players-versus-DM mentality that you really don't want to encourage in a game.

But, back to the point, the monsters could definitely retreat under fire and pull back into their own hallway position, or even leave the area entirely. Too often I see PCs playing with smart tactics while the monsters just growl and run at the party over and over. Only mindless creatures would do that; an obviously overmatched party of intelligent monsters should retreat, regroup, reinforce.

Engage on their own terms.

On the same basic level as above: Anything they can do, you can do. ("Better" is a matter of debate.)

There are very few monsters who totally lack ranged options. If the party likes to huddle in the back and lob spells, arrows, and cantrips, the monsters can absolutely respond in kind. Orcs can throw spears and axes just as well as they can charge in swinging. If the party insists on standing in a tightly clustered hallway position, use their tactic against them. Fireball and lightning bolt can be highly effective against a group that insists on standing in a straight line.

An extended artillery battle might not be much fun, but if they're gonna play games with doorways, you can play right back, and suddenly moving in close to prevent such a standoff seems a lot better.

By the way, keep in mind that you can take an action at any point during a move, so it's completely valid to have a lich standing to one side of the door, out of easy line-of-sight, then run into view, fire a spell off, and move back into cover on the other side of the doorway. They can ready attacks to hit him when he appears, but it's not a foolproof plan since there's also spells like invisibility and mirror image out there.

Enforce the rules

It's entirely likely that any direct attack made while trying to "shoot over your party's shoulders" in a cramped hallway would invoke a cover penalty on the guys in back. Based on the Dungeon Master's Guide rules about cover when using miniatures (DMG p.251, with diagrams on 250), it's hard to imagine any arrangement of characters as described that wouldn't have 3/4 cover against virtually any target they want to shoot at. They might be able to arrange to have merely half cover against some targets, depending on how you read the cover rules, but in general, firing between allies would be a lot like shooting through an arrow-slit, so 3/4 cover sounds right.

However, ranged attacks from the monsters would have the same penalty against the back-liners, so this isn't actually a solution as much as a way to make the party have less fun and even more dependent on area effects (where cover is measured from the origin point of the blast). Everyone getting a +5 to AC will make fights stretch on and on, and makes it feel bad to the players to keep rolling misses.

Spread out the monsters.

Both tactically and strategically. If the party is relying on throwing area effects, make sure you keep your monsters spread out so only a few can be affected at a time, and maybe break up a big fight into a few smaller fights where they'll have to blow through more spell slots if they want to keep doing their shtick. And along that line...

Don't let the party rest easy.

This strategy seems to me to be very spell-slot-intensive. They have to keep the front line healed, and get almost all their damage from big damage spells like fireball. That suggests to me that the party may be sleeping more than normal, and that you might be allowing them to go take a long rest any time they want to.

Pressure the party to hurry up with time sensitive missions. Launch ambushes if they sleep in the dungeon. If they leave the dungeon to rest, the monsters use that time to reinforce their numbers, reinhabit rooms previously cleared, set guards or traps, block doors or build barricades, summon demonic defenders, and so on. Make the monsters at least as tactically smart as the PCs are. Let the party know that relying almost entirely on the spellcasters' damage output is not going to cut it.

The module is only a suggestion.

The module should not be treated as a bible. Encounters can and should move around, DMs are encouraged to add or remove creatures to make the fights harder or easier, and so on. Tune your adventure to the party, if necessary. Don't feel constrained by the words on the page.

If they're depending heavily on spells for damage, counter magic with magic when you can, especially if they keep leaving the dungeon to "wait until tomorrow". The local spellcasters can and should adapt to their strategies by preparing protection from elements, wall spells, even globe of invulnerability if the levels are high enough. Keep in mind that a simple fog cloud can be utterly devastating to any ranged combat tactics, and a wall of fire blocks line of sight in addition to pulsing fire damage onto the party and forcing them to either back off or run through. Old standbys like grease and web are some additional fantastic low level options for making an area very undesirable to the party; personally I love web because of the irony of punishing the players for failing to move around by making it impossible to move around.