Can anybody explain me why is it that employers buy employees health insurance rather than having them buy it themselves?
Actually, it's an unintended consequence of socialism. Imagine that.
Back during the Great Depression in the 1930's, as part of FDR's policy of trying to control the economy, wages and prices were fixed. The few employers who were still competing for labor at the time couldn't offer any more in wages, so they searched for loopholes, and found they could add insurance benefits to entice the labor they wanted.
This kicked off the practice of insurance being through full-time employers being the main providers of healthcare coverage in the United States. In the multi-decade post-war boom, this wasn't much of a problem. However, because the consumers of the healthcare were divorced from the prices, we got into our current bind where we have a neurotic semi-free market where price/cost/availability curves of supply and demand and opportunity costs and utility are broken.
Consumers (until we started hitting the wall over the past 5-10 years anyway,) just use as much medical care as they could, with no prioritization or concern about costs. Hospitals, doctors, and medical clinics in America all compete on services, not on cost. They all compete by adding laboratories, MRI's fancy new clinic buildings etc. often with excess capacity as large corporate hospital and clinic chains try to out-saturate one another in local markets.
Depending on the insurance plan, the person sometimes pays just $20 out of pocket for a visit. Or, even if they have very limited coverage, the bill comes AFTER, and there is NEVER a discussion about what options in treatment you have or what the costs are. Also, medical debt is unsecured debt, and if it becomes overwhelming, they can sue you in civil court, and you just refuse to pay the judgment with no real penalty, other than your credit rating. If they apply for wage garnishments, you just file for bankruptcy.
Added to that, medical malpractice has forced American medicine into "standards of care" in that even if you wanted them, to pick from a menu of services for various rates of pay, against certain assumed risks to try and interject some market forces into medical care, there really aren't options anymore. There generally is one conservative or better-safe-than-sorry method of treatment that's the most lawsuit proof for most any medical condition you may have.
Insurance was being paid by employers with deeper pockets than individuals, and costs went up on the insurance side too. Until again, we hit the wall, and high deductible catastrophic care plans, coupled with tax-free medical savings accounts started becoming popular.
.gov medical plans for the retired or indigent, they set fixed schedules of payment for services sort of like the single-payer socialized medicine countries, except here there's no rationing, and the gap in costs are shunted over to the private side.
So in essence, it's a mucking fess, but better than the single-payer countries, because when you're dying, you can still get the care
now, and just worry about the bill later.