Why hasn’t humanity evolved such that every male had 20 cm (7.8 in) penises?

Oh dear, no one tell him he got the short end of the genes…

Joking, of course.

I would assume penises didn’t need to evolve to be that long on average since vaginas are not that deep on average? So there would only be so much depth to reach in the first place. Possibly girth would also affect it - having a long noodle dick might mean no matter how deep you go, the penis might not be wide enough to effectively “scoop” it out.

Again assuming, but after a certain length (bottoming out), width would become more important