Why do people say the 144,000 are only Jews?

There are 12 Tribes of Israel. Jacob whose name was changed to Israel after he wrestled with an angel had 12 sons who father a Nation of Hebrews. The Tribe of Dan is not sealed in Revelation 7. The Tribe of Manasseh takes it's place and Ephraim is properly called Joseph. Only 2 tribes of the 12 tribes are Jews (Judah and Benjmain) the Southern Kingdom. The number no man could number are the gentile believers. The people in the land of Canaan (Israel) are Revelation 2:9 and 3:9. Genesis 10 says Ashkenaz makes up the isles of the gentiles.