Personal thing really - I'm a solo flyer and prefer it to the discipline and compromise that comes with playing & singing in a band. Did a guest spot with an "Irish" band last night in front of approx 200 though and there's no denying the buzz is intoxicating.
There are many who play and sing in bands who wouldn't want to go solo as the "collective" gives them more confidence.

I'd love to play in a band, but have to admit I don't have the patience for the rehearsal/rehearsal/rehearsal (agree totally with comments above) and neither do I have the will to deal with the internal politics and democratisation required to agree on final musical outcomes. I recognise I'm missing out on something though; the genuine synergy that can be achieved when folks work well together.