It’s not the case for everyone; plenty of people have room for their third molars and a surprising number don’t grow some or all of their wisdom teeth to begin with. Why do so many people end up needing these teeth removed?
The leading theory about why we grow an extra set of molars that often doesn’t fit is the Soft Foods Theory. Basically, the modern diet of cooked and processed foods doesn’t stimulate as much jaw growth as what our prehistoric ancestors had to eat, so the jaw often doesn’t grow large enough for those teeth. The trade-off is that our teeth don’t wear out so fast from eating extra tough food all the time.
Wisdom teeth usually aren’t a problem for orthodontic patients. They aren’t often a factor in dental crowding, so we typically don’t remove them for the sake of the straightness of the other teeth. If the wisdom teeth do need to be removed due to being impacted or not erupting properly, that can be done while the braces are still on. Post-braces smiles won’t become crooked again because of wisdom teeth (but they will if the patient neglects their retainers!).