There's simple logic behind all these things. People want things to do. Either you give them something to do, or you let them make their own things for them and other people to do, or a combination of both.
If all they can do is high-five and kick people in the nuts and wander about a few landscapes and chat, that's not really enough for the first, and if you don't let them upload their own builds and clothes and objects (often because you're worried about them making a huge cock - which they would, immediately, it's the first thing that gets built, see Spore creature videos) you don't get the second.