I thought they existed because eons ago humans lost molars in young age, so there was room for those third molars when they came in? With modern dental care, if you keep all your teeth there isn't room for the third ones?
Before that we practically didn't had caries (just like wild animals also don't).
Throughout human history humans would have the full set of teeth in the vast majority of cases, until we started settling and switched from hunter-gatherer to agricultural society (which is a very recent time in human species development).