Ummm.... no, they don't. I have a friend who works in the industry and you'd be surprised how cautious some physicians are. The decision to treat a patient with a new drug isn't "oh! it's better, let's use it!", it's much more complicated than that. Even great drugs need to be promoted.

Also, you can't compare the costs of marketing and R&D. You don't spend marketing dollars unless you think the return is greater than 1 (you make back more than you spent). If drug companies stopped marketing their drugs, they'd end up in a worse financial situation rather than a better one. That extra revenue means more R&D spend!