Thinking about this also gave me a couple other ideas!
1) One might say that while making his own music on a professional level is arduous, but why would he skip over singing backup on someone else's track when all he has to do is show up and learn the part? I think he only does that for his buddies or people he cares about (hence BB King, Laura Creamer, etc) but otherwise shrugs it off. Why should he run around lending his voice to everyone he asks, when he was probably just asked to increased their marketability?
As for singing with women, we actually had a thread dedicated to this about a year ago: "
Glenn Backing Up the Ladies." He's just not interested, I guess, and never has been. Of course, we're assuming he's been asked.
2) Maybe part of his reluctance to release new music is insecurity.
Strange Weather didn't do well, and he poured his heart into it. Maybe he just doesn't want to take that chance again.