There is absolutely no significant number of Americans who left without ties to other countries. I find it rich that Americans who leave the US call themselves “ex-pats” instead of “immigrants”
Brit’s will also call themselves expats. https://britishexpats.com/forum/ ditto Canadians https://www.expatden.com/global/canadians-living-abroad/ Also, the US imposes taxes on Americans who leave until they renounce their citizenship on the upside they still get to vote. It’s an unusual relationship to your former country.
If you're saying that people who have permanently left the US call themselves ex-pats, that is news to me, and I can understand the confusion.