Here's why:

* Plateau Definition: A plateau is a large, flat area of land elevated above the surrounding area. It's characterized by relatively level or gently sloping terrain.

* Mountain Definition: A mountain is a landform that rises significantly above the surrounding terrain, typically with a defined peak or summit.

While the Ethiopian Plateau has many mountains within its boundaries, the plateau itself is a vast, elevated region with a generally flat or gently undulating topography.

It's important to remember that geographical terms can have nuances, and sometimes there can be overlapping features.