Generally, they offer a giant infrastruture that no other game selling platform offers:
- cloud saving
- community hub for sharing user content
- inbuilt forum
- mod database
- better shop visibility through intelligent algorithms
- community events like NEXT fest
overall, you get what you pay for.
Alternatively tgey could use the Rimworld model: release DLCs that heavily change how you play the game, allowing you to tailor the game to your wants and needs that way.