Put yourself in their shoes. It's going to cost you $50MM-500MM just to test if a drug is effective and safe for treating a disease.
You sink all of that money in, with the hopes that your drug gets approved. And then, once you've done the heavy lifting, every generic manufacturer can come along and sell the drug because you don't have it patented?
You must consider though how the massive test run costs associated only exist because the FDA believed drug companies could foot the bill since they got monopoly distribution.
In a patent-free world, those regulations wouldn't hold weight. What would happen (I imagine) is independently sponsored r&d produces drugs for trial that the FDA itself puts through clinical trials on taxpayer money. You can't sell a drug without FDA approval still, but since drug companies aren't operating as super-for-profit businesses on chemical compounds to help people live, they would foot the bill to make sure its safe.
You sink all of that money in, with the hopes that your drug gets approved. And then, once you've done the heavy lifting, every generic manufacturer can come along and sell the drug because you don't have it patented?
No rational person/company will do that.