Granted, the US president has quite some power. But he still does not rule alone (see Trump impeachment).
Remember Obama and single payer health care? Why did it not happen?
The answer is (according to Obamas own words): It would mean 3 million people in the insurance industry losing their job.
But a public option very nearly
did pass, which would have also (IMO) meant the death of the private medical insurance industry, just in a slower and more drawn-out fashion.
If your point here is that single-payer is politically difficult because of those 3 million job cuts, then I agree (but a strong economy with close to 0% unemployment like the one we have now is the best possible time to make that transition, so that those people can find work somewhere else).
If your point is that it should not be done because of those 3 million jobs, then I disagree. The free market can and should be steered in directions that are actually beneficial to society. The medical insurance industry is not only not-beneficial, it is actively harmful and we should do better.