He doesn't do it because he's barely longer than the next few guys up, and he knows the real number is nowhere near what some might expect.
What I've said before.. he's roughly the length of wrist to elbow on a girl. That's 9 - 9.5".
Also, look at his belly button. To his belly button is around 7.5" and he's a good 1.5" past it... approaching 2" but not quite. Again, that comes out at 9 - 9.5".
So that's 2 ways to get a good idea and they both come out around the same.
He's also around 3 female hands worth. A female's hand is around 3 - 3.5. Again... putting him around 9 - 9.5".