This discussion has happened here before. Echoing roughly what I said then: It happens in the US because it's the norm. Norm being synonymous with tradition.
I'm wondering what are the psychological effects of this operation are...
Do you strongly associate it with being an American male?
If so, then might it subconsciously strengthen your national identity?. ie, it helps you to feel American?
If so, might it strengthen feelings of patriotism?
On Independence Day during my 13th year on this Earth, my kinfolk presented me with my detached foreskin, preserved in a hermetically sealed box since the day of my birth.
To celebrate the rise of our young nation, and the transition from childhood to manhood, I stretched and fashioned my foreskin into an American flag; a mighty symbol of freedom that was as large as the 50 states of our fledgling republic in both length and breadth.
From that day forth, like so many who came before me, in both mind and body I knew that I was truly both a man and an American citizen.
That foreskin flag still flies from my front porch today, though it can get a bit crusty at times if you don't practice good hygiene and obey the flag code.