We baptize babies because we believe that God’s gifts given through baptism are for babies, too. In Baptism, God offers people of any age the gift of faith in Jesus, the forgiveness He earned for them on the cross, and the promise of eternal life with God forever.
We baptize babies because Jesus told us to.
Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you. And behold, I am with you always, to the end of the age. ~Matthew 28:19-20
And Peter said to them, “Repent and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins, and you will receive the gift of the Holy Spirit. For the promise is for you and for your children and for all who are far off, everyone whom the Lord our God calls to himself.” ~Acts 2:38-39
Some Christians believe that baptism should be reserved for those who can think for themselves and can make a conscious decision to believe in Jesus. We do not see baptism as something we do for God, but rather as something He does for us and in us, no matter how old we are.