Because performance is already a limit. There is no need to have another limiter. If people want to tweak every possible setting even if it's not recommended, let them do it. This is a PC game, not some sort of bad console port, so I think we should be able to.
Sometimes certain settings even downgrade automatically while you are playing, but that's not the right way to solve performance issues.
My suggestion is: remove the limiter and the auto-downgrade completely. And eventually add a pop-up warning screen which tells you that some settings are not recommended for your computer.