It started during WW II when the US government put wage and price controls in place so that companies could not compete for employees by offering higher wages. So they competed for employees instead by offering employer-paid healthcare as a benefit. Then after the war, when the wage and price controls were repealed, the employer-paid healthcare system, instead of going away, kept getting more elaborate.

As with a lot of things, such as vacation time, Americans seem to prefer to provide certain social goods as employer benefits because that way it seems more like a reward for competitive merit, which one can show off as a status symbol, than like a universal social good.