Because it's too hard for them. The standards are too high. It doesn't appease their egos and earthly ambitions.
It's also a much bigger "ask" for a woman than it is for a man. A traditional Catholic woman has to accept the possibility that she might end up having to bear, raise, and care for a large family if she marries, or else remain single for life. Having a "fun-sized" family, having her tubes tied when she's had the two or three that she wants, and juggling a career in the midst of all that, isn't a choice that traditional Catholicism offers, and for many women, I have to think that staying single isn't to their liking either. (And don't forget, the larger society almost universally accepts premarital sex. A reasonably attractive woman under a certain age can have all the sex she wants, so why settle for the same man forever? For men, it's not so easy.)
But it kind of cuts both ways. A man, especially in today's world, has to situate himself so that he would be able to support such a family, and quite frankly, not all men can do that, some can barely take care of themselves (and add to that the preference for women and various other demographics, that makes it harder for many men to find good jobs). Likewise, "we'll just have two or three, and we'll both work" isn't a choice.
There can always be exceptions (for instance, a woman who has some kind of physical disorder that prevents her from conceiving, either before having any children, or something that comes up afterwards), but I speak here of the general situation.