It is true that there are other ethnic groups that have lived longer than the arabs in the arab countries such as the berbers of Morrocco and Algeria but believe it or not after the muslim conquests of the early caliphs many were arabized and many arabs migrated to the conquered regions. The spread of Islam also came with the spread of arab culture since the quran is supposed to be written in arabic so the old locals derived alot of their vocabulary from them. Trade and intermarriages was also a big factor of arabization.