I think a lot of indie developers work alone because they want complete control over their projects. If they're like me, they've had problems with the whole design-by-committee model. Too many cooks spoiling the soup and such. Not to mention people want to make their own ideas come to life, not necessarily someone else's.
If I found a project I was seriously interested in, and I didn't already have projects of my own brewing, I might consider working with a small group of people who have well-defined roles. This would mean that a sprite maker is a sprite maker, a coder is a coder, a music person makes music, and they don't try to interfere with each other too much. All the team efforts I've ever been a part of were total chaos with people trying to take on ten different roles at once, and no one could agree on anything.
Regardless, I do have projects of my own going right now, and it's hard enough to find the time for those, let alone a collaborative endeavor. Good luck finding people, though.