It isn't that simple - investments and their returns have risks built in, and people expect more return for more risk. If you are an employee, you are not risking your own money, you are just losing some opportunities for cash. This lost opportunity does work into the equation, but not at the same risk level as an investor because if the company tanks, the investors lose every dime... But the employee still got paid the whole time.

I do agree that for negotiation purposes, considering pay below market value is a good point for discussion. But don't get offended if it is not weighted as heavily as people who actually wrote checks.