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

Not all men have small Penis

the average American has a Giant 5 inches mos

It is not important? Never has been. Next question, why isn't it erect all the time? Ans.: BEAR and you can't run as fast...so the long ones get "taken out" of the selective process of Evolution.(only the strong and fast survive)

. So blame God.  as it would have been "your golden calf" that you pay homage to.

Human males have the largest penises based on body size.

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. 

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. 

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

i gather you have little experience in this area.

They try harder much more pleasure than business.

Cos babies are not reproductive 

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.