Military systems do usually have extensive self-test functionality in place, but when it comes to things like ejection seats and canopy detonation circuits that

Military systems do usually have extensive self-test functionality in place, but when it comes to things like ejection seats and canopy detonation circuits that absolutely must fire reliably the more logic you have in between the higher the risk that it won't work when needed. Nor is self-testing 100% reliable, as counterfeit chips might work well enough during a self-test sequence on the ground but fail when exposed to the extreme temperatures and vibrations of an ejection. It's not just about whether the part meets the spec, it's about meeting the spec in all conditions. That's why for things like this and similarly crucial things like munitions-related circuitry the preference is for circuits and components that are as simple as possible and to use process control to ensure that all components are reliable.