They are never inferior to existing ones in all respects for one very obvious reason: allergies.
If you’re allergic to drug A but not drug B, it doesn’t matter how much better A is than B. You need drug B.
All drugs have the potential to cause allergic reactions or other nasty side effects so unless a drug is too dangerous on its own, it should be allowed. It’s absolutely critical to deal with biochemical diversity in humans.
Good point, but many of the me-too drugs are chemically close enough to the original to cross-react. (And in some cases are actually identical--a company finds a compound the body will convert into the original drug.)