There is a "pass the test of time" factor involved. If natural selection approves, then you surely can be more confident and even pay less attention because time guaranteed that plenty of diverse cases didn't compromised the survivability of the system and the system needs you less to achieve its success rate as designed.

Then it comes innovation. Which necessarily starts with limited cases. And we use to launch business and products as soon as we can. Hence, with the minimum acceptable cases. This is fine as long as failures cause minor reparable degradations instead of catastrophic unsurvivable cases.

And still, we need systems designed with every layer having the feature of graceful degradations. That enables us to perform as Woody and Buzz Lightyear said: "Falling with style"