Here's why:

* Erosion: Buttes are formed by erosion, primarily from wind and water. The softer rock surrounding a harder, more resistant layer erodes away, leaving the harder layer standing tall.

* Resistant Caprock: The top of the butte is often composed of a resistant type of rock like sandstone or granite. This caprock protects the underlying layers from further erosion, resulting in a flat surface.

While the top of a butte is generally flat, it can also have some slight variations in shape depending on the specific geological processes that formed it.