Sincere question: would you give up your right to quit "on a whim" if your employer gave up their right to fire you "on a whim"?
For example, in 2012, at Timeout, Aksel Van der Wal was promoted to CEO. He had previously been the CTO, and now we needed a new CTO. We made an offer to a person who had a great reputation as an engineering manager at another company. But he was on a contract he could not get out of, nor would his employer let go of him. So we had to wait 9 months, while he finished that contract, before he could join us.