It's the same as how although you can occasionally get a "natural" person over seven feet tall, it's very rare. Most really tall people have gigantism or some other form of pituitary abnormality, and I believe every recorded person to break eight feet has had such an abnormality.

It's like asking why we've never had someone with 500 IQ. There's a hard limit where anyone with enough of a mutation to enable that would probably also not survive long at all. And indeed with the height thing we see a lot of super-tall people dying younger due to cardiovascular strain and other issues. You get the picture.