I also think that, if you look harder, you'll see that those coming here for treatment are mostly from countries that don't have modern health systems. I doubt many Dutch people come to the US for treatment.

The US is characterized by very high pinnacles of expertise- think Mayo, Cleveland Clinic, etc etc. Wealthy people who want the very best may travel to the US to access the best possible care. You're right, there are pinnacles in other countries too but they tend to have Socialized systems meaning you can't just fly in with your checkbook.

IME the Western European nations have extremely good care overall. I'm not competent to comment on the relative frequency of centres of excellence compared to the US- though I think the US would be higher- but that's not necessarily relevant to care available to Joe Average anyway.