For the same reason as there are specs for every single thing on the aircraft, be they systems or structure, and also inspections and maintenance intervals for most of them.
Initial design specs are to get as good a component as you can get for a reasonable price, weight, etc. (Note - not the best part possible).
Inspections and maintenance are there to catch failures before they pose a threat with a reasonably high - but not certain - degree of success.
Setting the spec thresholds higher usually allows either fewer findings at inspection or longer inspection intervals, but by and large it's the inspection and maintenance intervals which assure safety, not the initial spec. With the acknowledged exception of items lifed for the aircraft design life by the initial spec, though those don't always end up lasting the life.