Earlier quoted context omitted.
Assuming you're referring to the general development and supply of all drugs (in particular new drugs, as well as generics) the simple answer is that the process of discovering and developing new drugs is both high-risk and also heinously expensive. As such, whoever approaches it needs to have many billions to spend and --very likely-- lose. The current capitalist Pharma model has developed to suit: investors in Phar…
Research from the Pew Charitable Trusts shows that in 2012 alone 27 Billion dollars was spent by US Pharmaceutical companies to promote their drugs to doctors and patients. http://www.pewtrusts.org/en/research-and-analysis/fact-sheet... This is money that could have been spent on R&D instead of advertising if we moved to a non-profit pharmaceuticals model.
Let's assume, for a moment, that the people running these Pharma companies aren't stupid. We can therefore assume that they will mostly make sensible decisions on how to spend money. It therefore follows that a company calculates that by spending $X on promotion, they'll recoup $X+$Y, where Y is the additional revenue thanks to their promotion, over and above what they'd have made without promotion - i.e, their promotional spend is a worthwhile investment for them.
For each company individually, therefore, promotional spend can be reasonably assumed to generate greater revenue and profit, some of which will be used to fund research into new medicines.
Now, like you, I'd much rather live in a world where there was no promotional spend, and correspondingly lower drug prices... but that's not the current world. And pointing a finger --'but, marketing, sales reps'-- at promotion is not the full story.