Why aren't employers required to provide health care insurance to their employees?

[Mrs T]

Nothing is being said about the millions of employers that do not subscribe to some kind of health care insurance for their employees, even if part of it has to come out of the employees wages. I worked for a physician and the answer was that most women have husbands that have health care at their jobs. Shouldn't this be illegal?
Group insurance policies usually are much less costly than individual insurance!