Thats because thats how banks make money when a bank lends you money it lends you the money of the people, then when you pay your dept they keep the interest

Because they are working for profit

The national bank decides the max interest rate for lending and min interest rate for saving based on many factors including inflation rate, foreign policy and economic situations. After that banks will decide based on the above criterion make their best rate which will make them profitable.

First they don't set the interest for lending themselves, the central bank sets the limit on interest to control the rate of inflation.

Because they are greedy the speard is more than two digits in Ethiopia...