Because unlike at supermarkets or gas stations almost no one pays the "standard" price - their insurance company pays whatever rate they've negotiated with the hospital.
Notice that the EO doesn't require insurance companies to post the rates they've negotiated with the providers.
Screw the damn insurance companies.
What’s the cost in materials and labor to set a broken arm? The procedure is done a million times each year. There’s your baseline average price. Publish it. Hospitals can charge more or less. It allows consumers to compare prices between hospital A and hospital B. Competition is good for consumers.