Because most uncut people like their penis the way it is, while seemingly (atleast on this forum) cut people want to change theirs.
While I'm not knocking Penis Enlargement either (it has it's good points and downfalls, complications like anything else), I am simply stating the claims on why its easier for an uncut guy.
As an uncut guy, you gain more by even having the foreskin forward and gripping the head, thus, you gain more foreskin as well which means atleast a little more pleasure. If I grip my own head and pull it, it is as hard as if I have my foreskin forward. That is just my experience though, everyone is different.
Don't knock what you have that's better than those that don't.
There are probably many uncut guys who do Penis Enlargement, but of course, not a huge load of them come to this forum. You have to consider this is also more of an American forum. While routine circumcision on infants lowered to 50% in America, most of our generation in America had the unfortunate experience to be cut, thus why most people on this forum are cut.