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 "waking up" 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!