The evidence says so, that's why. Studies of men circumcised as teens or adults find a much higher rate of loss of sexual feeling up to total sexual dysfunction. Meanwhile, there has never been any study that reliably shows that men circumcised as infants suffered any loss. And, seeing as Jews are managing to reproduce without difficulty, it doesn't appear as there is any problem.
I believe that the reason for the difference is that newborn infants have not mapped their pleasure and pain receptors to specific parts of their body. They may know they're in pain, but they can't tell the source of the pain. My beliefs, however, may be wrong. The reliable evidence still stands.