We kind of have. Human penises are waaaay bigger than those of other primate species, and many other mammals (relative to overall body size).

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

Longer penises can scoop deeper in the couchhole

VANCE!

The longer the penis, the larger the fraction of women who will find sex with that person painful, and at some point the reproductive advantage of scooping out others' semen will be nullified due to the fact that no woman wants to sleep with you. I think I read that four inches is the length that most women find comfortable, but I don't remember where I read it.

couchhole

I love this.

Or make it up with more strokes, so longer intercourse.