Why the circumcision if God created the foreskin?

Ahmad Ibn Ali Ibn Abi Talib reported that Imam As-Sadiq (a.) was asked: “So teach me. Is there anything wrong with the creation and the arrangement of God?” He said: “No.” Whereupon they said: “Thus God created His creation without circumcision. Is this a wisdom of His, or a mere game?” He said: “No, it is more a wisdom of His.” Then they said to him: “You have changed the creation of God (4:119), and your work of circumcision of the foreskin is considered more right than that which God created for it. You find fault with the uncircumcised man even though God created him and you have praised circumcision even though it is your work, or are you saying that it was a mistake from God, not wisdom?” He said: “It is a wisdom from Allah, and correct, but He established that as a custom (Sunnah) and imposed it on His creation. Just as we found that the umbilical cord of a newborn baby, when it came out of its mother’s womb, was connected to its mother, so He created it, and He ordered the servants to cut it, and in leaving it there is clear corruption for the newborn and the mother, and so are the nails of a man. He ordered them to be cut when they grow longer and He was able to create them as a creation that does not grow longer on the day He regulated the creation of man, and the same is true of the hair of the moustache and head that grows longer and is cut off, and the same is true of the oxen. God made them male and yet their castration is more appropriate and in that there is no fault in the regulation of God.” [Al-Ihtijaj of At-Tabarsi, Volume 2, Page 76]

أحمد بن علي بن أبي طالب عن جعفر بن محمد الصادق عليهما السلام أنه سئل: فأخبرني هل يعاب شيء من خلق الله وتدبيره قال لا فقيل فإن الله خلق خلقه غرلا أذلك منه حكمة أم عبث قال: بل منه حكمة فقيل له غيرتم خلق الله وجعلتم فعلكم في قطع الغلفة أصوب مما خلق الله لها، وعبتم الأغلف والله خلقه ومدحتم الختان وهو فعلكم. أم تقولون أن ذلك من الله كان خطأ غير حكمة قال عليه السلام ذلك من الله حكمة وصواب غير أنه سن ذلك وأوجبه على خلقه كما أن المولود إذا خرج من بطن أمه وجدنا سرته متصلة بسرة أمه، كذلك خلقها الحكيم فأمر العباد بقطعها، وفي تركها فساد بين للمولود والأم وكذلك أظفار الإنسان أمر إذا طالت أن تقلم، وكان قادرا يوم دبر خلق الإنسان أن يخلقها خلقة لا تطول، وكذلك الشعر من الشارب والرأس، يطول فيجز، وكذلك الثيران خلقها الله فحولة، وإخصاؤها أوفق، وليس في ذلك عيب في تقدير الله عز وجل