Because with a lot of musicians they need the money now. Forget the ones that are playing to support their habits. You have to maintain your equipment which isn't cheap, get from studio to studio which takes gas and toll, eat while you're recording (which can be up to ten hours a day if the studio is booked for a day) and all types of other shit. One of the guys on my project has his own studio so he makes a bit of money on the side doing that. Another offers guitar and piano lessons and another repairs and mods guitars and guitar amps on the side.
So I look at it like this. Do you get a sack, try it and then pay the D-boy only if you got high?