Why does East Africa have few important rivers other than the Nile?

Eastern Africa is mostly arid and is covered in desert area.

Explanation:

The Nile river is the only major river in the East African region as it is a desert and the Nile is the only river that can be there.

Even as such, it is a geographical anomaly for such a huge river to exist in such unsuitable plains and have been responsible for society there.

The green valley that the Nile forms is fed by relatively fewer tributaries unlike many other big rivers in the world. It is a single stream that feeds most of the populations of that predominantly desert area.