Actually it's prohibited by many (amish for example, which is why we have 1402(g)(1) in the IRC), frowned on by many (though not as many as in the past), and was sometimes outlawed without religious basis in the past in some places.
Slavery had been around for ~11,000 years before it was prohibited, AND it served a useful purpose. That didn't make it right. I could list less loaded examples, but they're all the same logical argument.
Contracts only exist between WILLING participants. In this case, it's mandated, removing the voluntary portion of that. Legality is temporal, not absolute, and morality is utterly subjective. While it's not immoral TO YOU, it IS immoral to me.
It is an evil, destructive force, destabilizing economies (by preventing equilibrium between earnings and costs by providing a false potentiality of payment), and siphoning off earnings on a gambled actuarial table that is not certain (and is itself based upon subjective, often kited prices, rather than actual costs). Moreover it adds unnecessary layers to transactions, further increasing prices and points of failure. Finally, any private for-profit insurance increases wealth concentration and in the case of traded companies may open investments to undue market risks.
I get what care I can afford without insurance. I will continue to do so. I will NEVER participate in any form of insurance. EVER. I would see every living things on the planet dead and burned first, though in application it's most likely just going to be me.