People continue to play games they have 'beat' for a very long time when those games have enjoyable (even if repetitive), relaxing or creative 'loops'.

It would be easy enough to name a number of such games many of which, by player numbers, are still considered very popular. And in many cases 'simple' mechanics keep more players engaged for longer times then overly complex ones (Stardew Valley is a good example of the former). Maybe because they can be more relaxing?

Personally I play perma-death now with no rush to the end boss. I also throw in 'weapon specific' runs sometimes to change things up (like only knives or -gasp- no bows).

Each time I croak I try something new on the next run even if its just a new building footprint for my outposts.

It is a game you can step away from for whatever life reasons might come and then pick up again without ever losing your 'connection' to the game world and your avatar.

I imagine your avatar just happily goes about farming carrots, breeding boars and fending off Greydwarves while awaiting your return.