Looking at some streamers who know the game inside out (and modded Minecraft as well), they get to a point where they can comfortably build whatever they want, build it, then start over. In vanilla it's usually some sort of mega-build with lavish bases and transportation systems. In modded Minecraft it's all about getting crazy overpowered gear, automation, efficiency and optimizations (sometimes necessary, sometimes just because). Check out Agrarian Skies LPs and other challenge maps or quest minded modpacks (Crash Landing was pretty popular last time I looked) to see what that can mean taken to its most grindiest, challenging conclusion.
All of that is done in survival mode of course, because it's more satisfying to them to mine and fend off monsters as they build. Actually being able to shape the world is simply really, really compelling to some people.
