Because I don't think there is a max price. Like of course there is in practice case by case because individuals don't have infinite money but nobody wants to be told "sorry, we can treat your condition but we're not going to because you're not valuable enough to society to get it." The episode Critical Care in Voyager muses on what such a system looks like formalized and it's awful.

And I think what makes it so that we're resistant to caps is because it's not just rare diseases you could write off as unlikely to ever get that are ruinously expensive and it's likely that in everyone's social sphere they know multiple people personally who've had "blown out their out of pocket max by factors of 5-10x" medical issues. It's a this really can happen to you thing.

So I think if your goal is to reduce healthcare costs on a nation scale your only option is make it so your people develop health problems less and tackle the smaller but much much much more frequent expenses. Things like ending the caps on the number of doctors, giving nurse practitioners full prescribing rights, moving more medications OTC, ending drug patent loopholes or for critical medications or "buying out" the patient so it can be immediately be made generic, adding more restrictions to testing so doctors have to actually think before ordering every test under the sun because it's not their money, massively reducing the regulations on medical devices, I could go on forever.

Stupid unnecessary expense times your population is way more money than the treatment for some rare disease.