I think because the Minecraft world is like a giant tub of Lego. You go out into the procedural world to collect your pieces and then you create from that. It's actually your creations that are interesting and without them Minecraft would equally purposeless and bland.

Multiplayer, demographic, community, and probably because it mainstreamed the genre. I'm pretty sure people, especially early teens, like to socialize on Minecraft with their friends. There is also the pseudo campaign of defeating the ender dragon. Minecraft is a juggernaut.

I suspect it does apply to Minecraft too, however Minecraft also benefits from its multiplayer communities in modding scene.