Why do women need so much prep time before they will have sex?

This is an in general question pertaining to all women. Why do women require what I call build up? Why aren’t they just simply ready for sex like men are? I don’t mean to say all women are like this. I am sure there are women out there that are ready for sex at the drop of a hat. Good for the man that you are with, but I mean your average woman. What’s the deal ladies? Why does it have to be this way?