String theory isn't very well supported evidentially. Or it is, but not in the way that people think.
Higher dimensions exist in string theory because after the tried to make the math work, various physicists couldn't so one possible solution was to add higher dimensions to explain where a bunch of hidden remainders went. It would be a little like adding 2 + 2 then getting 9 so saying that the answer in our 3 spatial dimensions is 4 but the 5 reminder is spread into their own very tightly wound undetectable dimensions.
String theory is wrong. We know this already. However what the real answer is will look a lot like string theory in some parts so it's useful to keep studying.