Why are the same drugs, manufactured by the same companies, marketed under different trade names?

That's to cut down on international sales of drugs. If you're looking for ABC, and you can only find a drug called DEF called in another country, then you probably won't recognize it as ABC in this country, and you won't buy it and try to import it into this country (legally or illegally). Drugs in America are sold at a tremendous markup over what they are marketed for in other countries. Thank lawyers and their lawsuits for that. Part of the price of every pill you buy pays for the lawyer who will eventually sue the drug company over that drug. Other countries don't have predatory lawyers like we do in America, so they don't have to put a huge markup on drugs to pay for lawsuits that do not exist. Hence, ABC costs you, say, $100 per bottle in America, but DEF, the exact same drug, sells for only $10 in, say, South Africa. It's all about money, and that usually involves lawyers.