I figure some of them just don't want to admit that they're missing a part of their "manhood." Maybe there's hidden resentment, maybe they don't want to admit their parents made a mistake, but at the end of the day they're hearing that others' penises are "more intact" than theirs, and that might be tough to accept.

I'm speculating to an extent - I don't have a penis, but it's not difficult to imagine the complicated feelings some circumcised men go through upon learning of what they've ~~lost~~ had taken from them, and that there's nothing they can do about it.

I wouldn't be surprised if the same thing happens to some women who've experienced FGM. They went through a horrific practice and can't come to terms that they might "have less" than others, so they double-down on the idea that what they went through must be okay.