Because some people have certain conditions preventing them to vaccines. For example, babies are too young for many vaccines. We still want herd immunity to protect those people. Vaccine effectiveness isn't 100%, so vaccinated people can still get affected by those irresponsible anti-vaxxers.
Antivax people don't just die, they overload hospitals. The more virus is spread, the more variants are created.
Those two are supported through evidence, but cannot be directly tested. To answer the OP's questions, shouldn't the theory be easily tested and repeated like thermodynamics?
Try using your PieFed account via Voyager:
https://vger.app/search
Somehow it is able to find results I couldn't directly with PieFed.