Comment by Enginerrrd

Because it's part of the arms race with pathogens that has been going on for a billion years since multi-cellular life first appeared. Pathogens do lots of very clever things to hide from the immune system, so the immune system is tuned to detect very subtle changes. The body largely operates on zero-trust network models, except for a few locations where the benefit of a too-active immune system has serious consequences to the fitness of the organism. The eyes and the testicles are good examples of that in humans.