Except it's not marketing products to doctors, who are the gatekeepers to access. It's marketing to consumers, who aren't informed, and are then used to badger doctors to make them prescribe medicines they heard about on TV.
Actually, big pharma does both. Marketing to doctors is sometimes even more sinister, because of (often implied) incentives for pushing more of a medication on the physician's patients.
And in reality, it's really only the US. Even though pharmaceutical ads are legal in NZ, they are pretty rare and most GPs won't just write a script because a patient asks for a particular drug unless there's a clear need for it. The role of Pharmac (NZ's central drug purchasing agency) also tempers the effectiveness of drug advertising so you just don't see it like you do in the mainstream US media.
