The reason Mormonism is not Christian is because it denies one or more of the of the fundamental doctrines of Christianity. These include that there is only one God, Jesus is God in flesh, forgiveness of sins is by grace alone, and Jesus rose from the dead physically, the gospel being the death, burial, and resurrection of Jesus. Mormonism denies three of them: how many gods there are, the person of Jesus, and His work of salvation.
Mormonism teaches that God the Father has a body of flesh and bones and that Jesus is a creation. It teaches that he was begotten in heaven as one of God’s spirit children (See the Book, Jesus the Christ, by James Talmage, p.8). This is in strict contrast to the biblical teaching that he is God in flesh (John 1:1,14), eternal (John 1:1,2,15), uncreated, yet born on earth (Col. 1:15), and the creator of all things (John 1:3; Col. 1:16-17). Jesus cannot be both created and not created at the same time.
Though Mormonism teaches that Jesus is God in flesh, it teaches that he is “a” god in flesh, one of three gods that comprise the office of the Trinity (Articles of Faith, by Talmage, pp. 35-40). These three gods are the Father, the Son, and the Holy Spirit. They do not accept the truth of the “Godhead” with there being only one God (Isaiah 44:6,8; 4:5).
Because Mormonism denies the biblical truth of who God is, who Jesus is, how forgiveness of sins is attained, and what the gospel is, the Mormon is not Christian — in spite of all his claims that he is Christian.
It is interesting that you do not find Christians calling themselves Mormons; but you find Mormons calling themselves Christians. Could it just be the politically correct thing to do?