As Dr. Gautam suggested, physicians are of a certain type, and most physicians are A-type personalities, perfectionists, hard-driven, want to get everything completed and perfectly so, which does lead to some of the 60-hour work weeks you're talking about.
I also wanted to mention that certain types of practice--and this may be why women are gravitating towards certain types of practices--lend themselves more to a communal type of practice with back-up. Certainly if we focus on where I've been focusing, on the surgical specialities, there is a special contract, or whatever you want to call it, between the patient and their surgeon. That's not as easily shared around.