The fundamental reason is pretty straightforward.
USA health care is designed to maximize profit. Hence everything should cost as much as possible. That’s why you get $1000/month insulin, for instance.
A bonus screw up, back in the 1970s there were various wage freezes, so employers started adding health care benefits in lieu of wages, to attract employees. This became extremely widespread. And objectively it is a terrible terrible terrible idea in every single way.
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