The fundamental reason why Jehovah's Witnesses teach that the body of Jesus was never resurrected and what came out of the tomb was a spirit creature who took on the appearance of Jesus, wounds and all, is their denial that the body of Jesus was resurrected and ascended into heaven. All this stuff about the pre-human Jesus being created by Jehovah as Michael the archangel and Jesus the man being dead, forever dead, has come about because they have adopted a form of Arianism, the heretical teaching of a 4th Century Catholic that Jesus was created.
Here is what they say about the death and resurrection of Jesus:
"What happened to the perfect fleshly body of Jesus after his death? Was it preserved so that in time men will look upon it in worship? Or does Jesus still have this fleshly body in the heavens, “spiritualized” so that it can be seen and worshiped? Neither. The Scriptures answer: It was disposed of by Jehovah God, dissolved into its constituent elements or atoms." Watchtower, 1 September 1953 (but no scripture says that)
"After conversing with those amazed disciples, the resurrected Jesus vanished. He dematerialized or dissolved that clothed human body. He did not take that body and its clothing into the spirit realm with him. Had it been possible to do so, it would mean that a spirit person in heaven does have flesh and bones, at least in the case of the glorified Jesus Christ. 1 Corinthians 15:50" Holy Spirit Active in the Invisible Heavenly Realm Chapter 2, page 19 (1976)
Nowhere in the Bible does it say any such thing. Psalm 16:11 says God would not allow his Holy One to see decay, and this is explained fully in Acts 2:22-32. The MAN, Jesus of Nazareth, who died, was raised up from the dead by God. God did not allow the BODY of Jesus to see decay because he resurrected his body from the grave. Not a ghost, not a spirit creature, not something or someone else.
A resurrection means that what dies comes back to life. A reincarnation means that what dies comes back to life in a different form. Jehovah's Witnesses would have you believe that Jesus, the man is dead, forever dead, and after his body "disappeared," what came to life was Michael the archangel. If Jesus was not physically resurrected, then there is no resurrection hope - for anyone, but especially for Jehovah's Witnesses who think billions of people will have a physical resurrection to life on this earth. Where is the evidence, the proof of the resurrection hope if Jesus' body just disappeared? As a Christian, I know that my Redeemer lives. And my Redeemer is not an archangel.
You are absolutely correct when you say this false teaching denies the resurrection of our Lord and if their teachings do not come from the Bible, then they come from that liar and deceiver, Satan.