Because if you don’t like your healthcare in the US, you certainly can leave, but you will be stuck with the bill, and if it’s considered leaving “against medical advice,” you will be denied insurance coverage if you return for the same issue.

Insurance is just a pain in general in the U.S. For instance some people might struggle to find a new doctor in their area if they're dealing with a specialist and have specific insurance coverages which means cutting off their toxic doctor might be more difficult.

Burning bridges to escape toxicity is fine, just don't strand yourself.