My experience of singing, as an actor, was that there's a different creative feeling of freedom. The acting thing is a bit more defined and cerebral. I can see why people would want to cross over. If you have so much freedom on stage then perhaps you want to be confined a bit, and vice versa.
I tell people that stand-up's like golf: you gotta do it every day to get it down - or at least three times a week to get it down.