True, they're not perfect, but the rub is that they claim they are. They claim they are the only true religion because only they are doing everything right.
You can't claim to be the only true religion whose leaders are inspired by God (a high standard) and then in the next breath claim imperfection as the reason they got things wrong. And, oops, sorry about that, folks
So, if they admit to being imperfect, then there's nothing inspired coming from them that needs to be obeyed at all costs!