But this isn't a full explanation, is it? There are schemes such as Medicare for All that a large majority of Americans accept, yet they aren't implemented.

And even amongst the republican party almost the majority support Medicare for All, yet democrats don't support it, why?

There are plenty of people that would support an imagined Medicare for All with the same cost that the UK's NHS has and the same level of care of the best US insurance plans, minus the bureaucracy and struggle to get treatment approved. Unfortunately, whilst this sells well to voters, any actual concrete implementation runs up against the fact that you can't actually have both - the NHS gets those low, low costs by not offering expensive treatments at all and having long waiting lists for most other things. Also, any major changes to US healthcare run into the issue that it involves a lot of well-paid jobs, and so a lot of the promises made by politicians along the lines of Medicare for All would involve making a lot of voters much less well-off by cutting their pay. That's not popular either.

People accept “Medicare for all” just like they accept the most recent “covid relief bill” with $1,400 checks per person. If you get into specifics and start including costs (taxes) into the equation, the popularity falls off a cliff.

Yes, I was so happy to get a $1,400 payment that costs me $5,000. Not that I didn't deposit the check....