When the test suite is too slow, it becomes unwieldy, it gets in the way (instead of being part of a valuable feedback loop) and everyone starts looking for shortcuts to avoid running it.
For sure. And I'd add that the specificity of unit tests is hugely valuable. If I do some sort of refactoring and found I've broken an end-to-end test, it may tell me very little about where the problem is. But the pattern of unit test breakage is very informative. Often just seeing the list of broken tests is enough to tell me what the issue is without even having to look at an error message.
