A couple of pieces of legislation in the 1940s and 1950s had the effect of encouraging companies to offer health benefits in lieu of others forms of compensation. Seems to have just snowballed from there.
Everyone agrees that it's terrible, but it's hard to change anything without screwing over at least one huge industry in some way, plus now the question is totally politicized and everyone assumes the worst about everyone else's intentions.
It is absurd; its roots lie in world war II wage controls and then once the interests were entrenched, it stuck. Plus a change in the rules on health care profits in, IIRC, the 1980s.
About half of the US gets government-sponsored healthcare (via Medicaid, Medicare, or one of the state-run programs). The other half don't, and getting it via your employer is generally a win-win (they are providing a benefit that costs them nothing [0], and you get to pay for healthcare via pre-tax income). You certainly don't have to rely on your employer for healthcare, you can buy any legal health insurance you like with your own money.
[0] Yes, it is extremely common for employers to pay for healthcare in full or in part (and the ACA complicates this a little), but there is no reason employers have to pay anything in order for employees to realize the tax benefit.