Because in their actual use, those terms cover most of the places Jews have lived: Jews from Eastern Europe, for example, are usually called Ashkenazi even though Poland is not Ashkenaz, and Jews from all of southern Europe are usually called Sephardic. North African Jews, properly called Mizrachi, are sometimes even lumped in with Sephardic.
Hi, there's lot of room for more depth on this topic but you'll find a few comments on other groups here