I've often wondered this too. It just seems so asinine that we tie healthcare to the employer--it's bad for employers and employees.
But even if it was accidental, it is morally perverse. For anyone with a serious illness, you can turn the statement around -- "You can stop working for me, but then you won't be able to get the medicine you need" -- and it sounds a lot like slavery.
One of the things that's perverse about it is that medical "insurance" is handled on a monthly basis, rather than on a per-diagnosis basis. Think about how insane that would be in another field. What if car insurance paid for things on a monthly basis rather than a per-accident basis? What if life insurance worked per-month rather than per-death?
My dream health insurance plan would insure me against the possibility of getting cancer -- and if I was diagnosed with it, it would be their responsibility to cover treatment, not just in any month in which I paid my premiums, but for the lifetime of any disease discovered while I was current on the plan. My dream health insurance plan would be willing to help me insure against variations in the cost of treatment for pre-existing conditions, but for conditions I truly couldn't pay for, would refer me to a charity.
I'm of the opinion that the misalignment is a result of businesses trying to keep employees healthy rather than the natural result of individuals mitigating risk.
The link between health insurance and employment needs to be severed yesterday.