It's how they get the results. They can only pinpoint specifics when more people of that ethnicity submit their DNA. Since a bunch of people in Asia and Africa don't give a shit about this stuff, they aren't submitting samples to build data off of. Your average Japanese person living in Japan, for example, maybe has some Korean, Filipino, or Chinese ancestry from a few generations back. But a lot of families have written records going back hundreds of years, so why would they want to pay for a test telling them something they already know?

There's a lot of Arab cultures like that, too, where they keep extensive records (orally or written) to the point they don't need DNA tests. I'm sure there's other places like that as well I'm just not familiar with.

It's really more of a western hemisphere thing where immigrants pretended to be from places they weren't or minorities lied about their ethnicities to avoid discrimination. Often you had Irish and Italians trying to pass off as French or English when coming through Ellis Island. Or people who were part black saying they were Native American or Greek so they could get into whites-only spaces.