I think you have evolution backwards. There only needs to not be a reason we need it to survive long enough to reproduce. Or more probabilistically, there needs to not be a significant reproductive benefit to it.

And bear in mind that most people don't have a problem surviving colds and the like long enough to reproduce even with no vaccines at all, and that was probably more true for much of our evolutionary history when we were living much more isolated lives, and not cohabiting with chickens and pigs.

There doesn’t need to be an evolutionary reason why we don’t have something. That’s the default!

Systemic cost.

We could have paper shredders, blenders, toasters, water taps, and so on that just ran all the time, but our utility bills would be ginormous. Same thing for our bodies.

Maybe it would made the immune system age faster if it is "used" too much.