Not everyone knows their worth and some people avoid confrontation (you are confronting your boss in a way). Some managers might overlook some people (not saying they are good/bad at managing if they do).

I had the experience of being underpaid (while I was still in school so it wasn't all about the money) but after I graduated I wanted to make at least close to what is going around here. I had it pretty lucky that I could just waltz into the boss's office and say I need to make more. It had been over a year and I had graduated in that time frame so I felt I had a bit of an argument and I am the guy who hates confrontation. At that point it was pretty much only about the money as almost everything else I did like.