In theory, if you had a competitive and fungible marketplace where a specific employee would boost profits by $100k, and you offered her a salary of $80k, someone else would outbid you with $90k, and someone else would outbid them at $95k etc. Even if the "margins" are low, no one is going to pass up on free money.
In practice, the labor marketplace isn't nearly competitive or fungible enough to expect the above. However, there is still a very different problem. Even if the average salary is "fair", people get rewarded a tiny fraction of the value they generate. Imagine if I'm working at Google and I find a way to save Google $10M/year via improved hardware efficiency. Despite the fact that this isn't part of my job description. How much of that $10M would I get in the form of a salary increase or bonus? A pretty darn tiny amount. 1% if I'm lucky. Is it worth it? Probably not. I might as well just do what my manager asks me to do, and spend my extra time learning Haskell and going to happy hours. This is pretty much what GP was alluding to. If companies started giving people a significant fraction of the value they generate, corporate life would change dramatically.