Maybe because they have to pay for college. Med school is about $50K/yr, maybe more. So now tell me what this has to do with the topic.
It's at the heart of this topic. You state that cutting Medicare reimbursements will result in fewer doctors willing to see Medicare patients. Why would they do that? Because they can make more money seeing patients with private insurance. So is the problem that Medicare is paying too little, or is it that private insurers are paying too much? I would argue that it's the latter.
The truth is that doctors are overpaid in this country relative to what doctors make everywhere else. That is one of the realities we need to face if we're ever going to get medical costs under control.
Doctors are underpaid compared to other occupations with similar education.
If you expect people to be rational, you aren't being rational.