Because it serves a genuine function, because the process poses an unnecessary risk, because there is no way to know how big the penis is going to get when the kid grows up, and that is part of the reason for the foreskin, to have a ton of give so it doesn’t happen like it did to my ex. He got circumcised as a newborn, and by the time he finished puberty, his penis grew far more than the leftover foreskin, so he wasn’t even able to have full erections without a tremendous amount of pain and sometimes, even tearing.