Well because the current modern Ethiopia with these borders are built by minilk before that everyone has an empire... Like there are always war between groups and the dominant one will expand its borders there are Islamic sultanes like Adal (from current harar to barbera somaliland) , yifat(around arsi), bale sultane, Hadiya sultane, Aussa sultane (afar), Abyssinain kingdom in the tigray and amahra regionregion) the kingdom of kafa, walaita, before 16 century the oromos were living under Gada system without living under neither the christian highland kingdoms or the islamic sultans that's why ...the current Ethiopia with 80 ethinc groups never lived under one emperor before minilk or maybe gragn ahmed