Every single time, from SN8 to SN11 (and feom what we've seen it's likely gonna happen on SN15 too) they needed to swap engines after static fires.

Why can't they detect those issues that they detect when the engines are mounted on the test vehicles at McGregor directly?

Either their test procedure/stand is not reproducing the environment (when mounted on SNx) well, or they lack quality control/SpaceX doesn't test enough, maybe they get damaged during transport to Boca Chica, maybe the materials don't withstand many relights or they even build the engines cheaper (less durable) because they only plan to fly them once or don't want expensive hardware to crash every time?

The correct answer is We don't know, but that doesn't mean we can't speculate about possible issues