Because writing good tests is very hard and many engineers are simply mediocre so they write brittle tests that require a lot of time to fix and don't actually

Because writing good tests is very hard and many engineers are simply mediocre so they write brittle tests that require a lot of time to fix and don't actually test the right things (e.g too many mocks) or simply overconfident (like some people in the thread) that their code will always work. Also the TDD cultists are partially to blame for this attitude as well. Instead of focusing on teaching people how to write valuable tests, they decided to preach dogma and that frustrated many engineers. I'm firmly in the circle of writing tests of course, I don't think a system that is not tested should ever be in production (and no, you opening your browser on a local machine to see if it works is not sufficient testing for production..).