The Merfolk Salvager’s melee attacks don’t specify that they’re “Weapon Attacks”. Is there a reason for this

While it's from Unearthed Arcana, the Close Quarters Shooter Fighting Style does this.

Close Quarters Shooter

You are trained in making ranged attacks at close quarters. When making a ranged attack while you are within 5 feet of a hostile creature , you do not have disadvantage on the attack roll . Your ranged attacks ignore half cover and three‑quarters cover against targets within 30 feet of you. Finally, you have a +1 bonus to attack rolls on ranged attacks.

Note that, for most spellcasters, you'll need a level of Fighter to get a fighting style. If you do, it's worth taking it first for extra proficiencies and proficiency in Constitution saving throws.

Be invisible. Yep, it's yet another way in which being invisible is awesome.

Aiming a ranged attack is more difficult when a foe is next to you. When you make a ranged attack with a weapon, a spell, or some other means, you have disadvantage on the attack roll if you are within 5 feet of a hostile creature who can see you and who isn’t incapacitated.

If you use regular Invisibility, it'll end as soon as you cast a spell, so you'll only get the lack of disadvantage on that single spell. Greater Invisibility allows you to overcome this and keep doing whatever you want.

As @J.A. Streich points out, any other way of being unseen will work just as well. For Warlocks with Devil Sight, Darkness is especially nice.

In 5E, the design standard seems be that almost all attacks - especially weapon attacks - feature a separate roll for each target. An “attack roll” in 5E is by convention a single attack vs a single target. Multiple target attacks generally have separate attack rolls made by the attacker (as in the case of multiattack actions), but if not, there's usually no attack roll and instead each target makes a saving throw to avoid some or all of the effect of the attack.

For melee attacks, there are two exceptions: the variant "Cleaving through Creatures" rules in the DMG and the Battle Master's Sweeping Attack manoeuvre, both of which allow an attacker to target a second creature as if making a second attack, but use the result of the initial attack roll. Both have limitations on when they can be used, though, and aside from reusing the initial roll, both are a second, separate attack. (Thanks to KorvinStarmast and Slagmoth for pointing these out.)

There are more examples of single attack vs multiple targets when it comes to spells: auto-hit spells like Magic Missile, which forgo a roll altogether (and as a result cannot critically hit), and unusual spells like Sleep or Color Spray which have the caster roll for a number of hit points of creatures who will be affected (but these spells don’t inflict damage).

An attack that hits more than one target with a single roll has several strengths and drawbacks that are not already present in the system:

• a single roll will be compared against multiple, potentially different armour classes - not gamebreaking, but it may remove some tactical options;
• it allows a creature to potentially score multiple critical hits with a single die roll - something not intended by the system;
• most importantly, there may be circumstances in which advantage or disadvantage applies against one or more targets, but not all - in which case, do you apply it to the roll or not? Rules for advantage and disadvantage may not allow for this case, and may become more or less powerful than intended depending on how you rule this.

I think these are all reasons why this was not a feature included in the game design, and so I’d be cautious about adding in your homebrew creations.