Not quite the question-- the real question is why would they be effective at the jobs of others? Doctors have 10-15 years of training, there's a reason for that.

They need a supervising physician, which should be an anesthesiologist. Surgical MDs do not have the same qualifications to sign off on their orders. The liability with CRNAs flows to the supervising physician, so most MDs tend to be very uncomfortable working with only CRNAs.