Why are almost all planes in the US? Is this a data problem or are only the US rich enough to fly private jets?

> The tracked set is built from FAA registry data

So I imagine planes in other countries exist, but the US FAA doesn't have data on them.

You wouldn't need FAA ownership data and details on who/what owns what tail number to track "flying" or "not flying" status of the world's fleet of bombardier global 6000/7000/8000, dassault falcon 7x and similar, given open data from ads-b exchange.