Because we know for a fact that boys in general don't die or lose their penises if their foreskin is intact. In other words, it is and was never necessary.
Ah. I understand how you are looking at it.
If it aint broke, don't fix it.
That's good, because it means you realize... hopefully, that man is not a product of blind processes, but a product of design.
Everything works as they should... right down to the foreskin.
However, your answer is like this answer...
"I saw a man slaughter 5,000 healthy sheep, and
it was not necessary, because all the sheep were walking about, eating, drinking, excreting, and bleating.
Sheep don't generally just drop down and die."
So the comparison - You don't know why the sheep were killed. You would have to get that information from the one who did the slaughtering.
There was a necessary reason... in this scenario.
You don't know it was not necessary to circumcise the Nation of Israel.
To know the reason, you would have to ask God.