The problem is not so much big companies wanting to lock in employees as the political nightmare of ending a tax-break. Right now, people who get employer based health care get a tax break: their compensation used to pay for health insurance doesn't get taxed. The more money you make, the more valuable that tax break is. People with employer based health care tend to be richer than people without. Every year, the government foregoes $80 billion in tax revenue by refusing to tax employer provided health benefits, and most of that money goes to middle and upper-middle class workers.

Congress isn't going to threaten an $80 billion/year tax cut that is almost entirely consumed by middle and upper middle class professionals.