Because he’s A) talking nonsense, and B) making genital mutilation of babies sound like it’s not that bad.
You want part of your dick cut off? Cool. Do it when you’re old enough to consent to it. Genuinely. If that’s what floats your boat, then do it. People should have the right to body autonomy. Slice your ear off as well if that’s what you’re into. Anything.
If this were about tattooing babies, or piercing them, or cutting off their earlobes, everyone would rightly be against it (even though all of these are less dangerous and less likely to have complications).
It’s only because Americans, Jews, and Muslims are used to this practice that they accept it. If it were a new thing, it would be seen as the barbaric practice that it truly is.
I believe there’s also the element of “well if I admit that circumcision of babies is wrong, that means there’s something wrong with my penis. And I don’t want there to be something wrong with my penis. Therefore I’m on the side of genital mutilation being ok.”