Partly because there are few if any mechanisms in place to actually generate it. But there are several factors.
People who are insured have no incentive to shop around for cheaper care.
Even people who aren't insured often don't have the *means* to shop around, as pricing for most procedures tends to be opaque, so people can't tell who is cheaper and who is more expensive. And, of course, people have limited capacity to shop around when they are dealing with a major medical problem, especially since once you're in a hospital it's difficult to go somewhere else for whatever treatments you need.
Providers are paid by the procedure, rather than for outcomes or number of patients, so they have an economic incentive to order more procedures than they actually need.
Litigation-fearing doctors may order completely unnecessary tests "just in case", because if they don't they may be sued in the tiny, remote chance that they're wrong.
Uninsured people are less likely to catch a potentially expensive medical problem (eg. skin cancer, diabetes) while it's still manageable, and thus end up needing expensive emergency treatment.
And, of course, the usual smattering of greed, fraud, and incompetence that can drive up the price of anything in a poorly-managed system.