Clearly the biggest "draw" for these sorts of games is new content, and that is a sort of sunk cost with developers who pay people to create new content so that they can monetize it.
However, when it shifts into something like an MLM scheme where an "end user" can develop new content and then deliver it through the game engine, and then the game engine owner and person who developed the content "share" in the monetization (one by game sales etc, the other by in game currencies) then games like PoE and others can grow diversity without hiring.
The incentive alignment is pretty clear, really avid players can both improve their own playing experience (enrich themselves in game) by doing this, and the game developer gets more people playing because there is such an interesting mix of things to do an see.
At some point this evolves into market exchanges where people who can conceive compelling stories find others who are skilled at art or puzzle design and they can form a group for the purposes of delivering a bit of gameplay content that they share in the rewards from.
If you are having a hard time visualizing this, consider WoW for a moment. Let's say Bliz had a way for an avid player to "design" an NPC, write their back story, their interaction dialogs, and the quests they would give in the execution of their "story". An artist teams up with this person to provide textures for either an existing character type, or potentially designing a completely new kind of creature for this. All WoW quests have a fairly fixed set of game mechanics choices to work from so putting those together in ways that are interesting might be the role of a third member of this ad-hoc group. Once completing the story line you have access to a vendor NPC who will sell you various items for WoW gold (pets, spells, recipes, etc). If the "team" that came up with this whole package got a cut of the WoW gold that the vendor received for their wares, that would provide an incentive to this team to make their content compelling. That may be all the incentive they need. And Bliz gets a quest chain + vendor + content that enhances game play for all players and keeps it fresh.
Sort of like the virtual gig economy where players add content to the game in exchange for in game currency.