in this case because they're serving in the US military and they need to be combat ready, and the state pays for their expenses.
also the claim about current vaccines is wrong. They do lower both risk of infection and transmission.
They don't do "essentially nothing" to prevent infection [1]. You are also at risk of both severe illness and long covid (something that appears to be under reported on) if you get it, even as a young healthy person. Where did you get your info?