Because I have nothing to prove, and no need to argue the point with anyone. My average girth is 5.5", and at the thickest point on a good day, I am 5.75".
Another issue I have with showing measurements on this site is that NO ONE accepts them. There are invariably people who will pick apart a measurement photo. Some one will mock the idea of a bone pressed measurement. Another will claim the ruler is tilted on purpose to show a bigger size. Yet another will argue shadows, hand sizes, whatever. Then of course there is the fake ruler (smaller size to make dick bigger) claim. Ad infinitum, ad nauseum. I do not make exorbitant claims. I know my size and report it accordingly. I do not care what people (such as the ever critical goldzilla) have to say.