Because the area Israel controls is covered with green while the land outside of Israel is mainly desert. This is true morally, culturally and physically.
Wikipedia says: The term Green Line is used to refer to the 1949 Armistice lines established between Israel and its neighbours (Egypt, Jordan, Lebanon and Syria) after the 1948 Arab-Israeli War.
According to Hebrew University Geographer Ilan Salomon, the Green Line can be discerned via satellite, marked by the Jewish National Fund pine forests planted to demarcate Israeli space.
