It is in part the result of policies designed to keep farmers on the land, which belongs to the state and cannot be sold. So farms are passed down the generations, divided and sub-divided. Many are so small and the land so overworked that it could not provide for the families that work it even with normal rainfall.
At present only 17% of Ethiopia’s 80 million people live in urban areas. Keeping people in the countryside is a way of preventing large-scale unemployment and the unrest that this might cause. BBC.