It's not even just that, but also that a lot of the things OSs do are painful to artificially test because they're on the edge of hardware and software, or involve building the abstractions that let other software run without worrying about those details. How do you make a CI job that tests that mdevd correctly handles enumerating devices and setting their /dev nodes correctly, when the edge cases are finicky hardware devices and nondeterministic enumeration?

So because you see testing all the things together as too prohibitive, you balk at testing the individual bits in isolation (hence "UNIT" testing)?