alright, so in general, I like 5e, it's got a lot of positives, but I'm noticing more and more that there aren't really any reasons for adventuring past a certain point.
So, an example: Our group is running some of the prefab content, specifically, we just finished up Dragon Heist (Which is really poorly named in a disappointing way), and are proceeding down into Undermountain, which is the assumed adventure track from the publications. The problem is…. why?
No, seriously, DMM doesn't explain any reason for you to dive into Undermountain, you just get a note that reads, "Come to The Yawning Portal. Undermountain beckons."
Even with Dragon Heist, there's not really too much reason to go along with the thing except for money, and the fact it's the adventure the DM bought and is now running. We actually got more into running the tavern…. until we got a taste of the RAW for running a business. It was boring, and even trying to invest to improve your roll didn't really benefit you, so it's just putting out money to get it handed back to you a moment later, or worse, it cuts into your profits rather than raising them.
Yeah, there are factions but the faction quests are never that interesting, which puts us back to adventuring, but if you do that, you'll hit retirement money pretty fast with no real need to spend the money on much of anything. I mean, any business will be mostly self-sustaining, and magic items have been made intentionally far more scarce across the board, most of which you don't even really need anyway, even out of those left over.
Unless you have a personal vendetta, there's no actual reason to do much adventuring past about level 5, if I'm reading it right. Past that, and most PCs I've seen should be getting ready to be that retired adventurer in a tavern, talking about how they took an arrow to the knee once.
So how can we work on this, and have a good solid reason to adventure that isn't some hackneyed thing about "something-something killed my parents" or whatnot?
Best Answer
If you are running DotMM, naked greed1
If your players don't like that, DotMM may not be the adventure that best fits your table's preferences. (More at the end of this answer1).
But by design ...
Dealing with threats to kingdoms and regions
The sequel to Waterdeep: Dragon Heist is tier 2 adventures. W:DH is based in the same setting as Dungeon of the Mad Mage, but your group is in no way a required follow on to that adventure. DotMM is a kind of dungeon crawl, which has its own internal logic if Dungeon Crawl is the kind of game you all want to play.
But that's not the only way to play.
Since your PCs are level 5, I'll suggest that you begin the Storm King's Thunder campaign. STK will take you to the end of tier 2 - the reason to keep on going is because there is an existential threat to the game world woven into the campaign. Or, you can give the players a hook and move on to the adventures in Tomb of Annihilation, which is underwritten by an existential threat to the world itself, and the souls of all PCs and NPCs in it.
Your players can be plugged into the SKT adventure almost seamlessly. ToA might be best to start from level 1, but you can start at level 5 and then try to end the threat to the game world by level 10, 11, or 12 based on what your players run into.
Embrace Tiers of Play as a framework
If you look in the early part of the PHB/Basic rules, you'll find this.
I have with one exception (lvl 20 campaign) played only one-shot adventures in Tier IV. Only that one campaign has lasted beyond level 15 at our tables.
Most play happens from levels 1-11.
One of the devs noted this in a tweet.
Why? Because at that point, the motivation for adventuring changes. Many DMs and many tables (and most published adventures) wrap up because multiverse-changing adventures are trickier to write and run than lower level stuff (through level 11).
Experience basis for this answer
Once you get to 5th level, adventuring is great fun. More skills, more spells, and harder enemies. I think it is fair to say that the sweet spot for heroic adventuring is from levels 5 to 11. The challenges and threats are at a regional or kingdom level, and each level brings a new skill or spell or feature.
If DotMM isn't doing that for you, then don't play it as a campaign. Use it as a "now and again" resource for a dungeon adventure with a specific magic item in that area as the goal. That means that the DM has to do some work to make it fit, contextually, into the campaign. Figure out which magic item in an area of DMM would be a good fit for that group, and then plant rumors and story hooks to entice the players to go there.
Good luck with Tier 3 content.
In tier 3, sixth and seventh level spells (like disintegrate and resurrection) are really powerful stuff - 8th and 9th level spells even moreso. I have played in two campaigns that went beyond level 11.
One ended in DM burnout at about level 14/15. We were trying to defeat the Giants in the Tales From the Yawning Portal "against the giants" package.
The other is alive and well at level 14 and we are beginning to change the world in the DM's homebrew world. His passion is world building. He keeps coming up with stuff that will challenge us or threaten our relatives, our businesses, our guilds, our nations, and the nature of the world. In one case there is a villain who is trying to replace the Grim Reaper, who just to happens to be the patron of our warlock. We may or may not succeed in thwarting those designs. (Update: Campaign ended at level 20 with PCs retiring, and a demigod forcibly retired / removed due to our party's actions).
Adventures at that level are tough to put together.
Your feeling that at level five adventurers are done does not square with my experience. My (me DM) Saltmarsh campaign is at level 7 heading to 8, and the shared world my brother and I DM has one group at level 9 and the other at level 6. Both are still going and there's much to do.
But at level 11?
A lot of published adventurers and settings reach their limit of interest somewhere in late tier 2.
1 The Fafhrd and the Grey Mouser answer: naked greed
Adventuring for the sake of adventuring: some players love to look for more loot, more XP, and more magic items and will play as long as they have those as possible rewards. DotMM is a good tool for that kind of game - adventuring for the sake of adventuring, the dungeon crawl taken to a logical extreme. But that's only one style of game.
As the Dungeon Masters Guide(DMG) p. 6 points out, there are a variety of ways for players to engage with the game. Take an inventory of your players. Talk to them. Find out what interests them.
The DMG provides a few insights on how to engage players regardless of whether they like Acting, Exploring, Instigating, Fighting, Optimizing, Problem Solving, or Storytelling best.
If you all don't like how DotMM fits into your group's preferences, try a different adventure, or make one of your own.