1) Because they are Christians or Christian influenced and this is the popular Chritian way of presenting things. However, if you study Judaism, you will realize that it has also evoluated since the New Testament. 2) "In general, Christians view Christianity as the fulfillment and successor of Judaism, and Christianity carried forward (and still does albeit in slightly modified form) much of the doctrine and many of the practices from that faith, including monotheism, the belief in a Messiah, and certain forms of worship (such as prayer, and reading from religious texts). Other beliefs around original sin atoned for by God giving his son, or the Son (who is God) coming down to earth for the sake of humanity, and a subsequent sacrifice of that Son, and the belief in the Trinity of God, are essential differences introduced in Christianity that have no counterpart in Judaism. Most Christians consider that the Law was necessary as an intermediate stage, but once the crucifixion of Jesus occurred, then adherence to civil and ceremonial Law was superseded by the New Covenant brought about by Christ's spiritual kingdom and His ultimate sacrifice upon the cross, respectively; the purpose of these laws was to dictate a proper relationship to God through the tabernacles and the temples in Jerusalem. Some Christians today, particularly of Roman Catholicand Calvinistic Reformed churches, hold to Replacement theology, the belief that the Jews' chosenness ended with Christ's sacrifice: Jews who remain non-Christian are considered to no longer be chosen, since they reject Jesus as the Messiah and Son of God. This position has been softened by some adherents, or completely rejected by some churches where Jews are recognized to have a special status due to their covenant with God the Father (through Abraham), so this continues to be an area of ongoing dispute among Christians. Some forms of Christianity (most notably Messianic Judaism) which view the Jewish people as close to God, seek to understand and incorporate elements of Jewish understanding or perspective into their Christian beliefs as a means to respect their "parent" religion or to more fully seek out and return to their Christian roots; messianics are sometimes compared to the Biblical Judaizers by fundamentalist critics because of the Judaic roots they seek to learn from. More evangelical Christians believe that many Jews have been judicially blinded by God so the gospel could be carried to the Gentiles, but Jews are also sometimes viewed as a people to whom Christians have a special obligation to evangelize. (See Missionaries)" Source and further information: http://en.wikipedia.org/wiki/Judaism_and_Christianity#Common_Christian_views_of_Judaism 3) However, modern views are more complex: "The curricula of Catholic seminaries and schools of theology should reflect the central importance of the church's new understanding of its relationship to Jews....Courses on Bible, patristics, early church history and liturgy should incorporate recent scholarship on Christian origins. Illumining the complex developments by which both the church and rabbinic Judaism emerged from early Judaism will establish a substantial foundation for ameliorating "the painful ignorance of the history and traditions of Judaism of which only negative aspects and often caricature seem to form part of the stock ideas of many Christians" (Notes on the Correct Way to Present Jews and Judaism in Catholic Preaching and Catechesis, #27, 1985)." Source and further information: http://en.wikipedia.org/wiki/Relations_between_Catholicism_and_Judaism

They don't know about the Talmud; and they don't know that Christianity was also a reform movement. By the time of Christ Judaism had changed a LOT. One of the biggest ways was that they had converted the spiritual Savior expected since Genesis 3:15 into a political messiah who would save them from Rome, restore the kingdom of David and Solomon, conquer the Gentiles, and leave every Jew a ruler over every Gentile. A popular doctrine, but not scriptural. That in fact was the biggest reason why Christ was crucified. On Palm Sunday they saw that he could lead an army--heal their wounded, raise their dead, feed them out of thin air and throw out the Romans--but he refused to. So they hated him.

Christianity is faith in Jesus as the promised Messiah and the absolute fulfillment of the Old Testament expectations.

Because without Jesus we'd be Jews, or trying to become Jews, and the most important document we'd have outlining God's will would be the Law that God handed down to Moses. As far as I know, this is the Law that the Jews follow but any Jews out there, feel free to tell me how and if I am wrong.

Bug in the Answerbag program (What are those guys smoking, anyhow?) that doesn't allow me to reply to your comment to my other answer, so here it is: Refuting your professor is not difficult but it requires lots of words. There are over 300 prophecies concerning the Messiah in the Old Testament, identifying him as a Jew from David's ancestry who would be born in Bethlehem when neither Judah nor David ruled in Jerusalem. This happened when Herod, an Idumean, took the throne. He would be like Moses--there are about 30 parallels--and would be badly disfigured in death (Isaiah 52:14), with hands and feet pierced (Psalm 22) buried with the rich but somehow still living (Isaiah 53). When Moses prophesied about him, he said "Listen to him." (Deuteronomy 18:15). He came preaching peace, not world conquest. The Jews didn't like to hear about pie in the sky by and by when you die; they wanted theirs down on the ground while you're still around. So they rejected him, and when after Palm Sunday he wouldn't give them the earthly power and freedom from Rome that he could have, they crucified him.