Probably because of the small population of people that today’s world population descends from, and the fact that mutation possibilities that are lethal or highly disadvantageous to survival don’t persist and are by default ruled out (core/important regions stay conserved between people).

The idea that a lot of our DNA is inert stems from when the human genome was sequenced, and they found out only a small percent of it codes for genes that go on to get translated into proteins. But, over time, scientists are finding that these non-coding regions of DNA don’t do nothing; in fact, they play many biological roles in regulating the expression of protein-coding genes and can have significant physiological impacts and relevance.