Because doctors and patients won't fix it. Doctors can charge whatever they want. They could see more patients, charge less, and charge cash. They could do their own diagnostics during the visit. People from other countries will be shocked to know that US doctors never do their own diagnostics, and instead refer patients to other providers, which require their own payment and paperwork, another round of appointments.

Putting the patient through all that for an ultrasound is "standard practice", which also happens to be maximally profitable for multiple providers and their associated office staffs, and the enormous industries that exist to service this deeply problematic "standard practice".