Part of it is that, in general, there aren't great options to spend 10 or 20 hours/week--or, really, 700 or so hours a year doing interesting work for someone else on your schedule getting paid at full-time professional salary levels. You're not available when needed. You're not keeping current probably. Even if I could do some part-time consulting on what I currently work on, I'd become much less interesting to hire pretty quickly.
