Because testing at this level is a likely impossible across all domains of programming. You can narrow the set of inputs and get relatively far, but the more complex the systems the broader the space of problems becomes. And even a simple crud app on an EC2 has a lot more failure modes than people are able to test for with current tools.

> passes a real test suite (not toy tests)

“not toy tests” is doing a lot of heavy lifting here. Like an immeasurable amount of lifting.