Why isn’t confession of sin included in the things required for salvation?

Answer:

The confession of sin preceded John's baptism (Matthew 3:6; Mark 1:5). While it might set precedence, it cannot be applied to Jesus' baptism without additional evidence.

Acts 19:18 refers to a confession of sin that took place by believers. "Many also of those who had believed kept coming, confessing and disclosing their practices" (Acts 19:18). The implication is that this was being done by Christians and not by people looking to become Christians. James 5:16 is of the same nature. James was addressed to Christians. He was telling Christians how to get help concerning sin. "Therefore, confess your sins to one another, and pray for one another so that you may be healed. The effective prayer of a righteous man can accomplish much" (James 5:16).

The purpose of confession is to face the fact that we sinned and that we are not going to hide from that fact. Unacknowledged sin destroys us internally. "When I kept silent, my bones grew old through my groaning all the day long. For day and night Your hand was heavy upon me; my vitality was turned into the drought of summer. I acknowledged my sin to You, and my iniquity I have not hidden. I said, "I will confess my transgressions to the LORD," and You forgave the iniquity of my sin." (Psalms 32:3-5). Confession is made to the ones we harmed by our sins. Since all sins are against God, the first and most important confession we need to make is to Him.

"This is the message which we have heard from him and announce to you, that God is light, and in him is no darkness at all. If we say that we have fellowship with him and walk in the darkness, we lie, and don't tell the truth. But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus Christ, his Son, cleanses us from all sin. If we say that we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, he is faithful and righteous to forgive us the sins, and to cleanse us from all unrighteousness. If we say that we haven't sinned, we make him a liar, and his word is not in us" (I John 1:5-10).

Often the one in sin realizes how badly his sin affected his relationship with God and so he desires the support of his fellow Christians in approaching God. You can see this with Simon.

"Now when Simon saw that the Holy Spirit was given through the laying on of the apostles' hands, he offered them money, saying, "Give me also this power, that whoever I lay my hands on may receive the Holy Spirit." But Peter said to him, "May your silver perish with you, because you thought you could obtain the gift of God with money! You have neither part nor lot in this matter, for your heart isn't right before God. Repent therefore of this, your wickedness, and ask God if perhaps the thought of your heart may be forgiven you. For I see that you are in the gall of bitterness and in the bondage of iniquity." Simon answered, "Pray for me to the Lord, that none of the things which you have spoken happen to me"" (Acts 8:18-24).

This is what James 5:16 was talking about. Acts 19:18 is another example of this even though praying for the sinners wasn't mentioned.

The confession of sin is an ongoing need since sin is an ever-present problem. Usually, we talk about the steps to salvation to discuss what is needed before being saved by Christ's blood. A long time ago, some unknown preacher selected five major requirements because it makes memorizing what is necessary to be saved easier to remember (you can count them off on your fingers). However, the Bible connects much more to salvation than just five things. See What Saves a Person?