There are a couple of possible explanations: Until the advent of modern dentistry, there was a very good chance that one would have lost multiple teeth by the time the wisdom teeth erupted, so there would have been room in the jaw to accommodate them, and they would have helped close the gaps. Also, our modern jaws are smaller that those of our ancestors. It’s just one of those bummers of evolution that our dentition didn’t change accordingly. If dentists hadn’t come along, then perhaps natural selection would have eventually weeded out the trait for wisdom teeth, but we’ve found our own technological fix for it so we don’t have to wait for that to happen.