Why would a creator create universally flawed bodies that need to be frequently changed? Is it a cultural issue rather than a strictly religious one?
When the Jewish nation was born, God asked Abraham for his foreskin as a sign that they had made an unbreakable covenant (promise) sealed with blood, and the circumcision of infant boys was evidence that the Jews adhered to that promise. It was a very private promise, hidden by clothing and only known to the man himself and his spouse. - It wasn't a public display like having particular cultic clothing, piercingsetc as some religious groups adopted.
It continues as the sign that the Jewish nation and God have never forgotten that promise. Islam sprang from Judaism (though much later). The Jews are the children of Isaac, Abraham's younger son, who was the son of God's promise and his legitimate child. The Arab nations are the sons of Ishmael, Abraham's elder son, who was born to a slave woman, Hagar. I assume that Muslims continue to circumcise for the same reason - that they believe God's promise.
When Christianity developed as a religion rather than as a small Jewish sect, Paul (Saul) - a Jew, and the author of a large portion of the NT stated that physical circumcision was unnecessary (Col 2.11 "In him you were also circumcised with a circumcision not performed by human hands. Your whole self ruled by the flesh was put off when you were circumcised by Christ" also Romans 2:29 "No, a man is a Jew because he is one inwardly, and circumcision is a matter of the heart, by the Spirit, not by the written code".). i.e. - the belief in Christ, and importantly following Christ's commandment to love one another and our enemies was a voluntary circumcision of the heart, as Christ had cut away the sin from his followers.
Jewish babies are circumcised on the 8th day, so that they have lived through an entire week which includes a Sabbath; I don't think that Muslims have a set age for circumcision, but would be happy for David to put me right on this as I do not know a lot about Islam (or Judaism tbh)