They send it to a ruthless collections company and ruin your credit.

That sounds like an entirely separate uniquely American problem. Don't you have laws that prevent disputed bills from affecting credit scores?

Edit: and wouldn't this lead to lawsuits against medical practitioners for extortion if it can be proved that they make a habit of doing this to patients?

No, because it is just an estimate. Every medical estimate in the US will include language that explains services other than those in the estimate may be provided, and you'll be responsible for paying for them. The provider is under no legal obligation to only perform the estimated services.

How is this legal? I mean, you can just straight up make it illegal. There's no fundamental law that says you have to allow such predatory practices.

There's a reason why health insurance companies do the amount of lobbying they do.

Also, in America, doctors will always have you sign an agreement before they see you and before the price is known, that essentially says, “you agree to be responsible for paying any amount the doctor asks for, without any limit.”

Surely goes to collections