Why do some Christians think baptism isn't essential?

It's true. According to the Bible, baptism is essential. Some Christians just believe what they want to believe regardless of what is in the scriptures.

Mark 16:16 He that believeth and is baptized shall be saved; but he that believeth not shall be damned.

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:

Oh dear, no. John 3:5 is not referring to baptism at all, you would have to use another passage. But you're half right, baptism is essential, not as an individual work, but as a corporate practice of the church that identifies its own through this practice. Think about it, can a new believer demand baptism, or is baptism only done with permission? Even in the Baptist churches, sure, you can let any sincere Christian baptize you, but we have to evaluate the form you fill out and tell you it's okay to do so.

There is only one verse that could link baptism and salvation. There are many that talk about our salvation not being predicated upon our works or deeds of righteousness.

Why are you making such a big thing out of a doctrine based on one verse? Shouldn't the main things be the main things? Did Jesus mention baptism when He separates the sheep from the goats on judgement day? Did he say that we most be born again and baptized to inherit the kingdom of God. Is baptism mentioned in the sermon on the mount? Does Paul mention a lack of baptism as something that keeps us from heaven? Romans 1, Col 1, don't mention it.

It all started with Martin Luther, when he came up with this "faith alone" doctrine (which was a misleading doctrine since the two words "faith" and "alone" are only found together in one place, and it says "not by faith alone"; Jas 2:24). When you look at his writings, you can tell he didn't mean to suggest that baptism wasn't essential to salvation, but that is what the doctrine eventually morphed into for a lot of people.

Before then, people always understood Jn 3:5 to be referring to water baptism. Below is a quote from Justin Martyr written only about 50 years or so years after John wrote his gospel.

I will also relate the manner in which we dedicated ourselves to God when we had been made new through Christ; lest, if we omit this, we seem to be unfair in the explanation we are making. As many as are persuaded and believe that what we teach and say is true, and undertake to be able to live accordingly, are instructed to pray and to entreat God with fasting, for the remission of their sins that are past, we praying and fasting with them. Then they are brought by us where there is water, and are regenerated in the same manner in which we were ourselves regenerated. For, in the name of God, the Father and Lord of the universe, and of our Savior Jesus Christ, and of the Holy Spirit, they then receive the washing with water. For Christ also said, “Except ye be born again, ye shall not enter into the kingdom of heaven.” (Chap. LXI - Christian Baptism, by Justin Martyr, AD 140).

When the gospel message of Jesus is taught like it was in the first century, people cannot wait to get baptized (Ac 8:35-36; 22:16). That's because biblical water baptism wasn't treated as a nice suggestion, but rather as a command (Ac 10:47-48). It was so important, that if someone was improperly baptized, they were immediately rebaptized properly (Ac 19:1-5).

For many, since their church teaches that way, they just accept it and don't question it (Ac 17:11).

Many baptism verses throughout the whole Bible are NOT wet verses!

Water baptism saves no one. It is as a consequence of salvation you go and get baptized. John 3

5 Jesus answered, “Most assuredly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God. 6 That which is born of the flesh is flesh, and that which is born of the Spirit is spirit. 7 Do not marvel that I said to you, ‘You must be born again.’ 8 The wind blows where it wishes, and you hear the sound of it, but cannot tell where it comes from and where it goes. So is everyone who is born of the Spirit.”

3:5 born of water and the Spirit. Jesus referred not to literal water here but to the need for “cleansing” (e.g., Eze 36:24–27). When water is used figuratively in the OT, it habitually refers to renewal or spiritual cleansing, especially when used in conjunction with “spirit” (Nu 19:17–19; Ps 51:9, 10; Is 32:15; 44:3–5; 55:1–3; Jer 2:13; Joel 2:28, 29). Thus, Jesus made reference to the spiritual washing or purification of the soul, accomplished by the Holy Spirit through the Word of God at the moment of salvation (cf. Eph 5:26; Tit 3:5), required for belonging to His kingdom.

3:8 The wind blows where it wishes. Jesus’ point was that just as the wind cannot be controlled or understood by human beings but its effects can be witnessed, so also it is with the Holy Spirit. He cannot be controlled or understood, but the proof of His work is apparent. Where the Spirit works, there is undeniable and unmistakable evidence.

MacArthur, J. F., Jr. (2006). The MacArthur study Bible: New American Standard Bible. (Jn 3:5–8). Nashville, TN: Thomas Nelson Publishers.

It helps when you provide the verse you are looking at.

John 3:5 then. Okay.

It's not talking about water baptism, in case you mean it does. It's talking about the baptism of the spirit in symbolic form. That is how I understand it. Water baptism is not going to save anyone. When the Spirit baptizes you, you know it, and God just took you as one of his own. Water baptism is an act of faith, which is an act of obedience. So when we heed to the directive of Christ about water baptism, we are put on the path of obedience, which is not the same as the act of faith trusting in Christ is supposed to be.

Romans 3 kjv

23 For all have sinned, and come short of the glory of God;

24 Being justified freely by his grace through the redemption that is in Christ Jesus:

25 Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God;

26 To declare, I say, at this time his righteousness: that he might be just, and the justifier of him which believeth in Jesus.

27 Where is boasting then? It is excluded. By what law? of works? Nay: but by the law of faith.

Salvation is a faith issue. You can't work your way into heaven, baptism is a work.

the whole idea about baptism is highly metaphorical and symbolic, since some Christian denominations actually practice baptism by submerging people in water

How do you know what the Savior said?

All you could possibly know is something that was written by several different men, then translated by still different men, then selected by still different men to be included in one of the dozens of versions of Bibles that exist today?

The most you can legitimately say is, "It says here that Jesus said xxx, and I have decided to believe what it says."

Show me where it says that it's a requirement to enter heaven