Because you're spending your days in class and your nights doing homework during your prime pairing years.

Men have the same problem a lot of the time -- a lot of engineers, doctors, and lawyers take some time spinning up on the runway with clear skies in front of them because they spent those formative years dedicated to their craft instead of meeting women. The big difference would be that humanity evolved with men being intrinsically worthless and so it's built in that they build worth later.

Imagine the brutality of a nihilistic postmodern world that says there is no such thing as worth. No wonder half of people will never breed.