1. Frequently you can't know ahead of time what is a good number to have. You can only pretend to know what is a good number, and people do pretend as if it can be done and everyone plays along with this bs.
2. You can get to that number in multitude of different ways, in many cases by not actually making the product better.
3. You might in the middle of progress realise that the path should be completely different, but now you are tied to that OKR.
They just don't make any sense at all, and at best they can take focus away from what should intuitively matter. "I know I should do X to actually benefit the product, but I do Y because this is what gets us to the Key Result.".
OKRs can be actively harmful because they take focus away from what matters, and you can't put a single number on those things.
OKRs when planning also focus you to plan poorly, again instead of figuring out how to conceptually make the product better, you set those arbitrary metric restrictions which can only stray you away from what you should actually plan to do.