Because you're leaving money on the table. For a profitable company, the employer isn't going to hire your services for more than they can sell it. But as an individual, obviously it's advantageous for you to capture as much of that value as possible.

Answer this: which makes you angrier? 1) You're making 100K when the product that you're the senior engineer for brings in 20 million for the company? OR 2) You're making 100K and Data Dan one desk over is making 110K?

It's all about what you anchor yourself to. Anchor yourself to everyone's else's salary, and awesome, you're making an average salary..