Why does evolution allow men to be born with small penises?

Because evolution doesn't breed out "unappealing" traits, only ones that inhibit the survival of a species. Having a small penis doesn't make a person any less or any more fertile, so genetics doesn't step in and correct it. 

Size of penis is not relevant to fertility.  Evolution is not a system with a goal.  It is not about "furthering the species".  Penis size is actually not something human women are generally considering when choosing a mate.

It’s not in any way a biological necessity to successfully find a mate. Most women will gladly favor a man who can provide wealth and security with a little dinger to a deadbeat unemployed guy with a donkey dong. 

"Size of penis is not relevant to fertility." You should have stopped there, Slickterp.

Evolutions IS a system with a goal, to make sure the species continues, hence us humans and most other species have evolved a strong sex drive. That is the only thing that matters here (plus the fertility), not shapes or sizes of anything else. It's more or less irrelevant.

i gather you have little experience in this area.

women dont know what they want until a man tells them

Human males have the largest penises based on body size.

They try harder much more pleasure than business.

Cos babies are not reproductive 

is mine small its only 7 1/2 inches long !!! Yes am white , not an asians ..or a black ...