A better question is why isn't agriculture nationalized? After all, food is even more fundamental and vital than healthcare, and by your logic is too important to be left in the hands of the private sector, "subjected to the PROFIT motive".
One answer is that we've seen what happens when food production is socialized, with millions dead from Soviet Ukraine to Ethiopia to Maoist China to the growing starvation in current Ukraine.
As for healthcare, what good is "coverage" if there's nothing to buy? The profit motive and relative freedom of the US industry has driven medical innovation for the world for over half a century. While that ends up benefiting the entire world, mitigating (and partially hiding) the failures of socialist healthcare systems, Americans have also enjoyed the best quality healthcare in the world, and a system with fewer waiting lines, more doctor/treatment choices, no government rationing, and by far more access to drugs. The problems we have are largely due to the clumsy government intervention we already have, from a skewed tax structure that pushed people from personal plans to employment based ones, to rigid regulations that limit choice and market flexibility (especially after Obamacare), to third party (government driven) interference in normal price signals that result in crazy unpredictability in cost per treatment.
But the bottom line here is that the vast majority of Americans have been satisfied with their private plans, as I said.
"As the incoming Congress prepares to debate further changes to the U.S. healthcare system, solid majorities of Americans rate the coverage (69%) and quality (80%) of the healthcare they personally receive as "excellent" or "good.""
Privately Insured, Rating "Good" or "Excellent"
Healthcare Quality - 85%
Healthcare Coverage - 70%