In flight test that’s what is done - it’s wrung out to find faults/weaknesses/strengths.
It’s no different than cars. You can test the thing a million times, but you cannot account for all possibilities. Take a look at cars, for example. Cars are only a fraction as complex as a jet fighters, but despite the battery of tests a car goes through before it goes into production, there is always something that becomes unaccounted for and a recall goes into effect.