Multiple issues with this logic.
First, developing a drug costs lots and lots and lots of money. Lots as in 9-10 digits. Why would someone spend a billion dollars developing a cure for a disease, but then not sell it?
Second, drug development trials are public domain. The US government even has a searchable website where you can go to read the reports on trials. If these cures existed, anyone would be able to look them up. They wouldn't be secrets.
Third, drug patents don't last very long. After looking up that cure that Merck or Pfizer inexplicably developed but never deployed, all another investor has to do is wait for the drug to come off patent and start producing it. Meaning that anyone or their mother can start a factory pumping out pills for said cure. Manufacturing drugs is dirt cheap. This is essentially the business model for Forest Laboratories.
Fourth, perhaps this is true, but it fails to take into account market strategy. Say three companies are making drugs that treat herpes. Then one company makes a cure for herpes. Perhaps that company would want to sit on the cure because the treatment is more profitable in the long run. But it's not nearly as profitable as the fact that this wonder drug would quickly gain a 100% market share and put its competitors out of business. Also, just because a disease has been cured doesn't mean it's going away. Strep throat can be cured. People still get strep throat and still buy antibiotics to treat it.
Source: scientist.