The boss can usually harm the subordinate's quality of life much more than the subordinate can harm the boss's quality of life. If you are fired than you immediately have less income. Your reputation is damaged if people learn of it. If you quit then it rarely causes any harm to the boss. They still have the same income. They probably have several other subordinates. Power imbalance is the defining characteristic of the boss/subordinate relationship. The boss can give orders and the subordinate will obey because the consequences if they refuse are worse for them than for the boss. This is what you pay to gain the stability of employment.
If I or most of my colleagues are fired we'll have less income for a month or two. Our lifestyle won't change [1] unless we want it to. In contrast, if my employees quit, my projects are significantly slowed, I need to spend a lot of effort hiring new ones (I hate hiring) and I'll look bad.
Furthermore, insofar as costs are unequal, that's merely a reflection of the principal/agent problem. Similarly, as a boss (but not an owner), I'm also motivated to overpay my workers; it keeps them happy and helps me get my work done (by hiring better people), and it's not my money that I'm spending.
[1] My colleagues all live in a country with a 30% savings rate and where American levels of financial recklessness are not socially acceptable.
