Why would an employee work for less than the value they are creating?
When two parties are negotiating a deal which creates value, both parties want to capture as big a share of that value as possible; the outcome will depend on the relative strength of their negotiating positions.
Let's say you have a machine that can make hot dogs for $3, and a concession at a sports event where you can sell hot dogs for $8. You're busy that day, so you hire me to take the cash and hand out the hot dogs.
The combination of your machine and my work generates $5 of value for each hot dog sold. If you capture all $5 I have no incentive to work; if I capture all $5 you have no incentive to lend me the machine. So how much will each of us capture?
If you have time and you can find some third party willing to do the work for $0.50 per hot dog you'll have a strong negotiating position - either I accept $0.50 per hot dog or you have the other guy do it.
On the other hand, if it's at the last minute and you can't find anyone, and there's another guy offering me $4 per hot dog, I'm in a much stronger negotiating position.
The amount of value an employee creates represents an upper bound on how much value they can expect to capture - job market competition provides the lower bound.