Hokay, first off, every male in this thread start lurking.

I'm joking, but in the end, ask a lady to answer a question about ladies.
Why sit in the corner? Honestly, as someone who doesn't do that, I can't give you too much of an answer. But, my theory is, go down this list and pick one:
1. They don't want to hang out with men. Seriously, they are not necessarily there to dance with guys, pick up guys, or even interact with guys. I would be thrilled to go to a dance with my lady-friends, and if we're together, I'm not going to suddenly dump them for some guy to dance with. I'm going to stay and interact.
2. They're self-conscious, maybe. They may like the club environment but don't know how to dance, or don't feel comfortable dancing, and they're just getting into things. Maybe they feel out of place. Maybe they just don't want to try it right now. Whatever it is, they have their own speed, so let them move at it.
3. They're picky. I.e. they will dance with a guy, but not you and/or your friends. They're looking maybe for a specific type, and it's not you.
This is mostly anecdata -- well, "what would my reasoning be," rather. So, if you want to find girls at a club, maybe you should look for the ones who are already participating, rather than angsting about the ones who aren't.