Which they tend to do a lot. Like, the moment a square root or trig function shows up.

Even without it's pretty easy to overflow a fraction stored the way you're describing. x=1/(x*x+1) does it in log time. There's really not a lot of situations where exact fractions work, but purely symbolic logic wouldn't. Maybe none, IDK.

Edit: I mean, I guess it's all symbolic. What I'm trying to say is that if you're at a level of detail where you can keep track of the size of denominators, native support for a type that hides them is counterproductive. It's better to lay out your program in such a way that you just use small integers, which is guaranteed to be possible in that case.