Why do some churches practice baby baptism?
Scholars disagree on the date when infant baptism was first practiced. Some believe that 1st-century Christians did not practice it, noting the lack of any explicit evidence of paedobaptism. Others, noting the lack of any explicit evidence of exclusion of paedobaptism, believe that they did, understanding biblical references to individuals "and [her] household" being baptised (Acts 16:15, Acts 16:31–33, 1 Corinthians 1:16) as including young children.
The earliest extra-biblical directions for baptism, which occur in the Didache (c. 100), are taken to be about baptism of adults, since they require fasting by the person to be baptised However, inscriptions dating back to the 2nd century which refer to young children as "children of God" may indicate that Christians customarily baptised infants too. The earliest reference to infant baptism was by Irenaeus (c.130–202) in his work Against Heresies. Due to its reference to Eleutherus as the current bishop of Rome, the work is usually dated c.180. Irenaeus speaks of children being "born again to God." This reference has been described as "obscure." Three passages by Origen (c.185–254) mention infant baptism as traditional and customary. While Tertullian writing c.198–203 advises the postponement of baptism of little children and the unmarried, he mentions that it was customary to baptise infants, with sponsors speaking on their behalf. The Apostolic Tradition, sometimes attributed to Hippolytus of Rome (died 235), describes how to perform the ceremony of baptism; it states that children were baptised first, and if any of them could not answer for themselves, their parents or someone else from their family was to answer for them.
From at least the 3rd century onward Christians baptised infants as standard practice, although some preferred to postpone baptism until late in life, so as to ensure forgiveness for all their preceding sins.
The problem with infant baptism (or christening) is the absence of belief and repentance necessary on the part of the child to validate the baptism. The child has committed no sins.
Mark 16:16 "He who believes and is baptized will be saved; but he who does not believe will be condemned.
Acts 2:38 Then Peter said to them, "Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins;
Baptism requires an understanding on the part of the recipient that it is the end of their old way of life and the beginning of a new, which is to be lived by the commandments of Jesus Christ.
Romans 6:4 Therefore we were buried with Him through baptism into death, that just as Christ was raised from the dead by the glory of the Father, even so we also should walk in newness of life.
Romans 13:14 But put on the Lord Jesus Christ, and make no provision for the flesh, to fulfill its lusts.
2 Corinthians 5:17 Therefore, if anyone is in Christ, he is a new creation; old things have passed away; behold, all things have become new.
None of these things mean anything to an infant.
Children are sanctified or ‘set apart’ by the belief and obedience to the faith of just one of their parents.
1 Corinthians 7:14 For the unbelieving husband is sanctified by the wife, and the unbelieving wife is sanctified by the husband; otherwise your children would be unclean, but now they are holy.
Immersion in water does nothing if one being baptised lacks a proper understanding of ‘the gospel of the kingdom of God and the name of Jesus Christ as taught by Jesus and his apostles (Acts 8:12). It does not matter who does the baptism, the baptism is validated only by the beliefs of the person being baptised.
Acts 8:36 Now as they went down the road, they came to some water. And the eunuch said, "See, here is water. What hinders me from being baptized?" 37 Then Philip said, "If you believe with all your heart, you may."
For this reason the disciples of John Baptist at Ephesus had to be rebaptised when they came to a proper understanding of the Gospel preached by Paul.
Acts 19:3 And he said to them, "Into what then were you baptized?" So they said, "Into John’s baptism." 4 Then Paul said, "John indeed baptized with a baptism of repentance, saying to the people that they should believe on Him who would come after him, that is, on Christ Jesus." 5 When they heard this, they were baptized in the name of the Lord Jesus.
Submitting to baptism has to be the ‘answer of a good conscience’.
1 Peter 3:21 The like figure whereunto even baptism doth also now save us (not the putting away of the filth of the flesh, but the answer of a good conscience toward God,) by the resurrection of Jesus Christ:
The baptism of infants serves no purpose with respect to their salvation.
I hope you find this helpful.