Because Saint Paul says baptism is for everyone!
Here is scripture in support for infant baptism:
Gen. 17:12, Lev. 12:3 - these texts show the circumcision of eight-day old babies as the way of entering into the Old Covenant - Col 2:11-12 - however, baptism is the new "circumcision" for all people of the New Covenant. Therefore, baptism is for babies as well as adults. God did not make His new Covenant narrower than the old Covenant. To the contrary, He made it wider, for both Jews and Gentiles, infants and adults.
Job 14:1-4 - man that is born of woman is full of trouble and unclean. Baptism is required for all human beings because of our sinful human nature.
Psalm 51:5 - we are conceived in the iniquity of sin. This shows the necessity of baptism from conception.
Matt. 18:2-5 - Jesus says unless we become like children, we cannot enter into heaven. So why would children be excluded from baptism?
Matt 19:14 - Jesus clearly says the kingdom of heaven also belongs to children. There is no age limit on entering the kingdom, and no age limit for being eligible for baptism.
Mark 10:14 - Jesus says to let the children come to Him for the kingdom of God also belongs to them. Jesus says nothing about being too young to come into the kingdom of God.
Mark 16:16 - Jesus says to the crowd, "He who believes and is baptized will be saved." But in reference to the same people, Jesus immediately follows with "He who does not believe will be condemned." This demonstrates that one can be baptized and still not be a believer. This disproves the Protestant argument that one must be a believer to be baptized. There is nothing in the Bible about a "believer's baptism."
Luke 18:15 - the people brought infants to Jesus that he might touch them. This proves that the receipt of grace is not dependent upon the age of reason.
Acts 2:38 - Peter says to the multitude, "Repent and be baptized.." Protestants use this verse to prove one must be a believer (not an infant) to be baptized. But the Greek translation literally says, "If you repent, then each one who is a part of you and yours must each be baptized." This is confirmed in the next verse.
Acts 2:39 - Peter then says baptism is specifically given to children as well as adults. God's covenant family includes children. The word "children" that Peter used comes from the Greek word "teknon" which also includes infants.
Luke 1:59 - this proves that "teknon" includes infants. Here, John as a "teknon" (infant) was circumcised. See also Acts 21:21. So baptism is for infants as well as adults.
Acts 10:47-48 - Peter baptized the entire house of Cornelius, which generally included infants and young children. There is not one word in Scripture about baptism being limited to adults.
Acts 16:15 - Paul baptized Lydia and her entire household. The word "household" comes from the Greek word "oikos" which is a household that includes infants and children.
Acts 16:15 - further, Paul baptizes the household based on Lydia's faith, not the faith of the members of the household. This demonstrates that parents can present their children for baptism based on the parents' faith, not the children's faith.
Acts 16:30-33 - it was only the adults who were candidates for baptism that had to profess a belief in Jesus. This is consistent with the Church's practice of instructing catechumens before baptism. But this verse does not support a "believer's baptism" requirement for everyone. See Acts 16:15,33.
Acts 16:33 - Paul baptized the jailer (an adult) and his entire household (which had to include children). Baptism is never limited to adults and those of the age of reason.
Rom. 5:12 - sin came through Adam and death through sin. Babies' souls are affected by Adam's sin and need baptism just like adult souls.
Rom. 5:15 - the grace of Jesus Christ surpasses that of the Old Covenant. So children can also enter the new Covenant in baptism. From a Jewish perspective, it would have been unthinkable to exclude infants and children from God's Covenant kingdom.
1 Cor. 1:16 - Paul baptized the household ("oikos") of Stephanus. Baptism is not limited to adults.
Eph. 1:1; Col. 1:2 - Paul addresses the "saints" of the Church, and these include the children he addresses in Eph. 6:1 and Col. 3:20. Children become saints of the Church only through baptism.
Eph. 2:3- we are all by nature children of wrath, in sin, like all mankind. Infants are no exception.
2 Thess. 3:10 - if anyone does not work let him not eat. But this implies that those who are unable to work should still be able to eat. Babies should not starve because they are unable to work, and should also not be denied baptism because they are unable to make a declaration of faith.
Matt. 9:2; Mark 2:3-5 - the faith of those who brought in the paralytic cured the paralytic's sins. This is an example of the forgiveness of sins based on another's faith, just like infant baptism. The infant child is forgiven of sin based on the parents' faith.
Matt. 8:5-13; Luke 6-10 - the servant is healed based upon the centurion's faith. This is another example of healing based on another's faith.
Mark 9:22-25 - Jesus exercises the child's unclean spirit based on the father's faith. This healing is again based on another's faith.
Exodus 12:24-28 - the Passover was based on the parent's faith. If they did not kill and eat the lamb, their first-born child died.
Joshua 5:2-7 - God punished Israel because the people had not circumcised their children. This was based on the parent's faith. The parents play a critical role in their child's salvation.