The US is really, really large and many people living there have never left it. There's also a belief that US healthcare is the gold standard (which is probably true at the very high end, but not necessarily on average).

In other words, it's a hard sell to convince a factory worker to travel 5-10 hours on a plane to a place they have no familiarity with to get what they believe to be inferior treatment. They might not even have a passport.

This is much less of an issue in Europe where countries are tiny and almost everyone travels between them on a regular basis.

Also, medical tourism is mostly limited to what the industry calls Shoppable Services - outpatient treatments that are not emergent/not life threatening. There is a bit of a cottage industry in Mexico for some services, like bariatric surgery for obesity or dental work, but it isn't a huge volume. The really expensive stuff in the US is emergency treatment which you can't shop for anyway.

All that said, I think the interest in buying prescriptions from overseas has never been higher.