It's not the only reason of course, but if a company is careful and consistently puts out quality products that are associated with that name, consumers have a pretty good idea what they are getting. If you buy a can of Coke in the US you know what it is. If you buy a counterfeit can of Coke from China you don't have a clue what's in it. That's a big reason why Chinese counterfeit products are such a big problem. Buy a fake Louis Vuitton bag and it will fall apart, but it won't kill you. Buy counterfeit airplane parts and you have an entirely different level of problems.

People buy what they trust.

I, on the other hand, buy store brand OTC meds.