Because demand and supply don't self-regulate healthcare. How much do you value your health? How about your own life? Oh, you're willing to pay ANYTHING to live? Even if it's not life threatening as long as it leaves you crippled, unable to work, you may as well be dead.
As a customer the only way to be an informed buyer is to be a physician yourself. Even if a treatment doesn't work you still get charged, no refunds!
For-profit healthcare is fundamentally inhumane and is incompatible with capitalism. The forces that would usually regulate the market are non-existant. Demand is infinite, undercutting is pointless, customers have no way to be informed.