It's because we build lousy software with lossy interfaces and strangely-undefined interactions that these tools exist. They're used when someone says "we'll never really know what's going on there, just throw some tooling at it and maybe we'll get lucky!"
One would hope we would try to make software more like the former than the latter.
Fuzzing, property testing and mutation testing are attempts to get an inherently complex system under control. Typically, these approaches are also cheaper than formal proofs to develop which is really important because engineering isn’t just building the thing. It’s evolving the thing, maintaining it, and the cost of all of this in manpower is absolutely critical to consider. Think Space Shuttle design where you have to get everything right up front vs SpaceX’s fail fast approach.
Only in the same sense where if people just wrote perfect bug free code then we would not have to test. Tragically, people are not omniscient or perfect programmers, so tools remain useful.
I think that sets an unnecessarily low upper bound on the complexity of the system.
Did Feynman completely understood the Space Shuttle when investigating the Challenger disaster? Perhaps, but we can't always rely on having a Nobel laureate at hand.