After WWII, with the start of widespread income tax, employers started giving health insurance to their employees as a way of giving a tax-free additional benefit.

Initially, it wasn't obvious this was a legal tax loophole, but the law was amended to explicitly allow it.

Now it's very hard to change, because getting health insurance from your employer makes you tied to your job. Even with the new health insurance exchanges, getting insurance through your employer is a pre-tax benefit, but if you buy on your own, it's with after-tax money.