Have you actually looked at how universal care works outside of the US in any of the countries that have it?

And by look I do not mean digest the usual ration of misrepresented horror stories that get repeated every so often.

About your question. There are several answers. A lot of it is related to paperwork overhead. A lot of it is legal. The fundamental problem is that when it is your life and death at stake, you will generally pay anything you own for care. That is the potential demand that the healthcare system is attempting to expand into...

Cheers,
Ben