Some did. The US expat community has been quite large for decades.
Most people didn’t do FIRE style early retirement while dealing with pre existing medical conditions. There however was plenty of expats pre ACA who very much left the country for early retirement.
US healthcare is ruinously expensive but on average it’s not particularly good if you’re in the income bracket where 1/4 million over a few years is a serious issue.
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”
There’s over 1/2 million former Americans living in Canada or the UK which doesn’t require learning a foreign language. You really can’t make those kinds of sweeping statements about populations that large. Many Americans without any prior connections fled to Canada to avoid the Vietnam war for example and then made a home there.
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.