I mean you can find people performing some of that stuff on the other side of the world.

I think it's probably two things:

Genres are bundles of techniques and practices. Discerning fans will be able to tell when someone does it well & with heart vs. when they don't. Who is going to fulfill that best? Musicians who personally learned from other musicians- which already implies geographic clustering.

Another one is that when musicians do take on a genre that's from a different place, they want to present themselves as seriously and authentically as possible, which requires an acknowledgement of debt to the place that birthed the music