This is a question that keyboard warriors throw around quite a bit.
Alongside, get in the cage and do your stuff on (Insert current Heavyweight Champ here)
The simple reason you don’t see it – is because they don’t know it.
Cast your mind back to the early days of the UFC.
Almost no-one knew any grappling / Ju Jitsu – so you did not see it.
Same thing… you will not see people doing something, that they don’t know how to do.
Not very often you will see a Boxer attempt a high kick… that kind of thing.
This video clip explains a bit more