Getting back to Danbo's original point of why some musicians are so prolific.
I remember reading an interview with Clive Nolan where he said he didn't really see himself as prolific, he said that music "is all I do". I suspect that is some cases, the mucisians prefer to offer their services without the ties of being part of a band on a permanent basis. They have the freedom to come and go, while being able to make a good living from doing what they love doing. If they don't want to do something they can just say they're not available.
There's also the benefit of not having to write the songs, promote the album, finance the project etc. While many artists will thrive on such pressures, others shy away from them.
Money of course will always come into it. We tend to assume that our heroes are wealthy beyond our dreams. The reality is of course that away from the headline acts, the income of many will be very variable, and at times almost non-existent.