Because they are afraid of the pain themselves and thinking about it? I'd imagine it's quite uncomfortable, all I think of is poor Al Bundy when he had his circumcision . I'm uncut and I had phimosis, it took a long time to stretch my foreskin so it would go behind the glans. I couldn't imagine having my cock head exposed 24x7 and basically be waiting for it to keratinize a bit and become desensitized. I hear in some other countries they do a "dorsal slit" where they slit the foreskin on the top instead of totally removing it.
I think the pain and thinking of the procedure might be part of it for some, I know I have the exact same reaction when I see a penis with peircings (PA especially)! my cock acts like it wants to retreat back into my body