This can be difficult to imagine until you've been there, but the last thing I'd want to do when terribly ill is travel, if it could be avoided. (That's assuming you're even fit to travel. Sometimes you aren't.) My chronic illness is unpredictable and disruptive enough to my family without us having to uproot our lives to deal with it in another setting for an unknown period of time, with doctors who may not know as much about it as a subspecialist closer to home.
It's not that I don't have faith in the healthcare systems of other countries, but some diseases and procedures are more common in certain parts of the world than others. Even within states or multi-state regions, there will be hospitals that direct specific patients to other hospitals for proper treatment. There are two sides to traveling for treatment, and people often focus on the more optimistic one, which is finding yourself in a situation so benign that you have the option to shop around and travel to maximize savings. The darker side is having to travel because it's your spouse's or child's only hope for survival.
There's so much variability around what a disease can take from you, how quickly (and painfully, and sometimes permanently) it might happen, and where to receive the best care for your situation. When I first got sick, as a toddler, with a common illness that had nothing to do with neglecting my health or making poor life choices, the bill was $200k, adjusted for inflation. That was several times what my parents' house cost back then. Many more hospitalizations followed. They sometimes had trouble locating doctors and surgeons with the right expertise to treat my level of disease severity. Thirty years later, my dad still gets emotional when he remembers the terror he felt when the bills arrived, and the relief when he saw insurance covered nearly everything.