Many major 3rd party libraries in Javascript either have their own global state, like React, or you interact with platform global state, like NodeJS `process` global (eg `process.on(signal, handler)`) or browser `window.navigator` or `window.document` globals. I have also heard plenty of horror stories about Java unit testing pain and suffering from developer tools engineers supporting Java, after all that's why Bazel is so focused on "hermetic" everything.

If I could just make some rules that all engineers would follow exactly, I'd simply make a rule "write no bugs" and then I wouldn't need a test framework at all.