Of course your tests shouldn't be sensitive to external state. Why would other tests running affect your test?

They shouldn't. But they do. We're not perfectly spherical developers and we all make mistakes. Sometimes it's also extremely tricky to figure out what state is leaking, especially is it's an access race issue and happens only for some tests and very rarely. If you haven't seen that happening, you just need to work on larger projects.

I've worked at several large companies tyvm. In our e2e tests we did not isolate dbs. If the test failed because of other tests running that's a bug in the test and that person would get automatically @mentioned on slack and they would have to fix the build.