Do you have any proof? A doctor once mentioned this to me, but then I talked to an orthodontist and he called it BS

I think the theory is that modern jaws are too narrow to contain all the teeth, causing them to overlap and grow in "unintended" directions. The reason for the narrowness is lack of exercise in childhood due to the soft nature of prepared foods. I'm also a bit sceptical.

Not proof, but logic: Without machinery to do the work of processing food, it has to done by hand or not at all, which leads to a lot more jaw work. Especially if you're eating a lot of animals where you'll be eating off of bones. But even with plants too.

Where is the feedback mechanism that ties extra jaw exercise to straight teeth?

The strengthening of the jaw during childhood leads to a larger jaw, and more room for teeth. Our jaws are too small. "Straightening" (like I said earlier) isn't exactly correct.

This whole topic is debated.