Because of the countless surveys that have been done on average size. This does not mean that someone can't have 10 friends that all have a 7" dick because anything is possible, but the probability that all your friends do is very small. This same type of possibility stands with women. Some woman may have had nothing but 10" dicks her whole life but the chance of this is slim. After reading through the countless surveys, my own research, the men who join these forums, etc. my only conclusion is that average size is about 5.5" x 4.75. I think that many men will never believe this for whatever reason but as far as surveys go, it is what it is.