[epivosism]

Think about how much you know about a random country, say Hungary. Probably only one composer, maybe one movie. But in Hungary there are millions of books, movies, etc most of which never left.

Same for humans during migrations. Only a small percent of humans left Africa, and then only a smaller percent kept going, etc. So as long as not much time has passed to generate new variation in the areas they settle, only a small fraction of local variation ever leaves is low. And that's what we see, Africa is extremely diverse, Europe/Central Asia less, east Asia even less, etc. It's neither bad nor good, just a number.

There are some other factors, such as mixing with people already present at the destination for a long time (Neandertals, Denisovans) which balance it out slightly.

The US has lots of diversity, but there were a lot of groups in Africa for a very long time (maybe 10k+ separate groups). It's unlikely that people from each one have made it here, or even out of Africa at all, in significant numbers.