> If the code is simple, the tests aren't much extra work.

Disagree. Even if it isn't much work to write (which is debatable), it still carries costs: CI takes longer to run, and the tests have to be carried and maintained (and possibly updated if the code itself changes).

> High test coverage doesn't mean your code is good, but it at least reduces the rate at which you accidentally break stuff.

Sure, for the most part. But that doesn't mean "more test code" is always a good thing. Tests aren't free, even for simple code.