The obvious argument is that natural organisms are restricted in ways that artificial ones are not (e.g. natural viruses are not selected to kill their host too quickly, natural selection can only reach solutions which can be reached incrementally).

>EDIT: Also, in this world, couldn’t an AI just “one shot” a vaccine and an effective treatment?

This does not follow automatically. E.g. it is enormously easier to create an ICBM than to create something capable of intercepting an ICBM. Some technologies naturally benefit an attacker and some naturally benefit a defender.