Why don’t Unitarians believe that Jesus Christ pre-existed before incarnation? I want to ask this again but in a different context. In the context of unitarianism. 


Why Unitarians do not believe that Jesus Christ is God
And why Unitarians do not believe in the pre-existence of Jesus prior to the incarnation. 
And where do Unitarians stand regarding the belief of non-pre-existence of Jesus Christ before birth in relation to the Bible verse John 14:6 which states:



  Jesus answered, "I am the way and the truth and the life. No one comes
  to the Father except through me. (NIV)


If Christ did not pre-exist before the incarnation, how did the people before the incarnation (Adam and Eve, the prophets, the patriarchs and just about anyone before Jesus' birth) who where all considered holy and have communicated to God in one way or another, were able to do so, when John 14:6 clearly states no one can come to the Father (God) except through Jesus Christ? 

Furthermore, Matthew 22:43-44 states (which is a direct reference to Psalm 110:1)  


  He[Jesus] said to them, "How is it then that David, speaking by the Spirit,
  calls him 'Lord'? For he says, "'The Lord said to my Lord: "Sit at my right hand until I put your enemies under your feet."'


In the non-unitarian context this is a clear evidence that Jesus pre-existed and an answer in the referred question linked on the top of this question, suggests:


  All of Scripture (the written Word) testifies of Jesus (the Living
  Word) and there has never been access to God the Father except through
  Him:
  
  You search the Scriptures because you think that in them you have
  eternal life; and it is they that bear witness about me, yet you
  refuse to come to me that you may have life. - John 5:39-40


Although this needs more explaination from the author [Mike Borden]

So, what is the view of a Unitarian about this?
The difficulty in trying to answer the question WHY is that only a Unitarian could possibly know.  There is no problem in explaining WHAT they believe with regard to the person of Jesus, but as to why they deny that Jesus pre-existed before coming to earth, then any explanation given by a Trinitarian, for example, would be denied by a Unitarian.  Let’s start with WHAT they believe and from that we may discover the WHY part:
Unitarianism teaches that God is simply one (unity). Unitarianism rejects the doctrines of the Trinity and the deity of Christ.  Unitarians teach that Jesus was an inspired man, a great teacher, and an example to follow but he certainly is not God.

Today there are a number of Unitarian groups throughout the United States and the world; probably the most well-known and influential group is the Unitarian Universalist Church. Unitarianism is usually accompanied by (or perhaps inspired by) rationalism and anti-supernaturalism. In the final analysis, Unitarians reject the specific teaching of Scripture regarding the deity of Christ and the Holy Spirit. Beyond that, they usually reject the Bible as divine revelation in favor of human reason. Horizontal relationships between human beings are considered more important than the vertical relationship between a holy God and sinful man.  https://www.gotquestions.org/unitarianism.html
To the Unitarian Universalist, Jesus was a good moral teacher, but nothing more. He is not considered to be divine, and every miracle associated with Him is rejected as being outside of human reason. Most sayings of Jesus recorded in the Bible are regarded as embellishments on the part of the authors. Among the Universalist beliefs: Jesus did not die to save mankind from sin, as man is not a fallen sinner; emphasis is placed on humankind’s capacity for goodness; sin is completely relative, and the term itself is rarely used; man saves himself through personal improvement, salvation being a purely worldly experience, a &quot;waking up&quot; to the world around oneself. This is very important, for death is final. Most Unitarian Universalists deny the existence of an afterlife, so all we have on earth is all we’ll ever get.  https://www.gotquestions.org/unitarian-universalism.html

From this we can deduce that Unitarian Universalists place their own reasoning above the teachings of the Bible.  Since they reject the Bible as God’s inspired word, then they are unlikely to grasp who the biblical Jesus really is.  Their Jesus appears to be a man-made concept, designed to fit in with their own ideas.
Then we have Biblical Unitarians who believe that the Bible is the source of truth.  They represent the more “conservative” branch of Unitarianism and many beliefs of Biblical Unitarians are in keeping with orthodox Christian beliefs. However, they depart from orthodoxy on one major point in regards to their doctrine of God.

Biblical Unitarians deny the Trinity, teaching that God is one being (hence the word Unitarian in their name). Jesus, according to Biblical Unitarianism, is not the eternal Son of God; rather, He was created by God in the womb of Mary. Jesus was later exalted by God and given authority over creation, making Him like God, but He remains a finite, separate being with a beginning.  https://www.gotquestions.org/Biblical-Unitarianism.html

WHY do they reject what the Bible tells us about Jesus being the eternal Son of God?  It all boils down to their view that the One Being of God must exclude the eternal co-existence of Jesus.
Jesus said this to the religious leaders who refused to acknowledge who he REALLY was:

You search the Scriptures because you think that in them you have eternal life; and it is they that bear witness about me, yet you refuse to come to me that you may have life (John 5:39-40).

It would be interesting to get an answer from a Unitarian that defended their position with regard to Jesus being created, because I would really like to know WHY they hold that view. I have my own opinion, but I'm keeping that to myself!