I think it's that they have already embraced the idea that maleness and femaleness are simply are simply aesthetic values with nothing more significant attached to them. Not real categories.
The idea of a male priesthood, at least in Christianity, is very attached to embodiment. It's why it tends to be found in sacramental churches - the kind of discussion you get about women in the ministry is around very different issues. Depending on whether you are talking about the western or eastern sacramental tradition, it's usually described in terms Christ, or the idea of fatherhood, both of which are understood as being about embodiment in real and substantial categories, which are fundamental to what it is to be human, and which are in themselves good. There is not some abstract sexless person that is more ideally human than actual sexed people.
The argument for an all male priesthood isn't like, women can't be good leaders etc. It's more analogous to, men can't be mothers. People like Welby don't accept, and generally in my experience don't even understand, that kind of argument. There are a surprising number of Anglican vicars who quite like the idea that Christ had a sexless body, despite it being a pretty ancient heresy.