The short answer is "Because that is not what the Bible teaches". Water baptism by immersion is only for those who have responded to the Gospel through repentance and believing on the Lord Jesus Christ (Mt 28:18-20; Mk 16:15,16). Which means that they must be capable of seeing themselves as sinners and Christ as their Savior and Lord. And sinners must repent and be converted.
As to circumcision, it was meant to symbolize repentance, conversion and faith, so one again it cannot be applied to babies.
And he [Abraham] received the sign of circumcision, a seal of the righteousness of the faith which he had yet being uncircumcised: that he might be the father of all them that believe, though they be not circumcised; that righteousness might be imputed unto them also (Rom 4:11).
But he is a Jew, which is one inwardly; and circumcision is that of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God. (Rom 2:29).
As to circumcision, it was meant to symbolize repentance, conversion and faith, so one again it cannot be applied to babies.
And he [Abraham] received the sign of circumcision, a seal of the righteousness of the faith which he had yet being uncircumcised: that he might be the father of all them that believe, though they be not circumcised; that righteousness might be imputed unto them also (Rom 4:11).
But he is a Jew, which is one inwardly; and circumcision is that of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God. (Rom 2:29).
Gen. 17:12 For the gerations to come every make among you who is eight days old must be circumcised, including those born in your household or bought with money foreigner-those who are not your offspring.