You know those little checks you do to make sure things are properly secured? Rattling the door handle after locking it, flipping your water bottle before putting it in your bag, testing your brakes before you leave the driveway?
If you’re not checking every failure case, how can you know you’re good to go? You don’t. So you need to check every failure case.
Not that it isn’t a pain. Test coverage is definitely a pain. But at least it’s useful when your tooling for it is.