[megameter]

Unit tests help verify individual components of a system - which makes them top-of-mind for library code.

I think the issue with them lies in that most developers aren't shipping libraries, they're shipping integrated systems, so there's no component worth testing. (you can always invent one, but that's just overcomplicating the code).

At the same time, it's also genuinely hard to write good, principled tests of integrated systems, harder than it is to code up a thing that kinda-works and then manually debugging it enough to ship. You have to have the system set up to be tested, and feature complexity actively resists this - you fight a losing battle against "YOLO code" that gets the effect at the expense of going around the test paradigm.