Because the largest chunk of healthcare delivery expenditures is for chronic and severe or emergency procedures for which people have no ability to shop around for due to the time sensitivity of the issue, or because the complexity and presentation of their health issue is not understood enough to put any price on.

For example, you go into the hospital for abdominal pain, but it turns out that you need a massive orthopedic surgery to fix a congenital issue that only 2 surgeons in your area can perform (true story). Or you are a severe diabetes or lupus sufferer who has routine hospitalizations for a variety of infections you get at random due to their underlying condition. Need a tooth filling replaced? Of course, shop around. Need emergency dental surgery because you had traumatic facial impact playing soccer? Not so much. Emergency treatments to sick geriatric patients to extend their lives by weeks and months: also very expensive.

Routine doctor's office visits to remove a wart from your foot, or diagnose a sore throat, which you can shop around for, are not where the high costs comes from.