It is true that providing people a pure sandbox with no GM involvement tends to leave people to quit the moment they lose steam.  That's been my experience.  People can write stuff about the setting any time they want; the promise of something interesting that the GM has in mind for the setting, and experiencing that, is what often keeps people engaged in an RP.  To me, it's reminiscent of video games.  Games like Minecraft can have an interesting premise, but without any sort of central narrative or real engagement with the world, a lot of people can get bored.

Personally, I find it fun to make stuff up in a sandbox setting, particularly if other people can experience it, but I can see how a hands-off approach be analogous to neglect, [@IceHeart].  What do you think?  Also, you totally got me on the 'important locations' list.  I somehow didn't register that it would be for player-made stuff as well.

Come to think of it, if you're looking for a co-GM to help provide more stuff to engage players with, I'd be interested in lending a hand.