Thank your local HMO or insurance company for that. It is them who say what doctors can or cannot do. medication is often seen as a more economically 'viable' alternative to inpatient treatment (esp. in case of mental disorders).
If the organization of the health case system allowed having a primary health care providers who keeps all your and your family records and, the problem of not checking patient history would be pretty much reduced, if not practically eliminated. However, that is practically impossible in the US, unless you are rich and can afford to pay for a doctors of your choice.
Most people have insurance through their work, and insurance companies force you to use their 'network' practitioners. Each time you change a job, your employer changes insurance company, or your current insurer jacks up the premium so you have to look for another insurer to be able to pay your rent -- you are forced to use a different 'network' doctor. So much for 'choosing your own doctor' the health business hacks are drooling so much.
In short it is the totally fucked up organization of health 'care' - or rather racket - in the US rather than doctor incompetence that is most dangerous threat to our health.
wojtek