Why does a plateau such as the Colorado Plateau have a flat top instead of jagged peaks?

Answer: Its level rock layers were lifted as one block without crumpling

Its level rock layers were lifted as one block without crumpling ✓ — The Colorado Plateau is a stack of sedimentary layers that were laid down flat, and when the region was pushed up, by as much as 3 kilometers, it rose largely as a single block with very little folding or faulting. The rocks of the nearby Rocky Mountains were crumpled as they rose, but here the layers stayed level, so the land that sits on them is flat too, apart from the canyons rivers have since cut into it.

Floods of lava spread across it and hardened into a level surface — Lava did not build the Colorado Plateau, which is made mostly of layered sedimentary rock such as sandstone and limestone. Stacked lava floods did build other flat highlands, including the Columbia Plateau in the north-western United States and the Deccan in India.

Glaciers ground its mountain peaks down to a single height — Glaciers did not plane the Colorado Plateau flat: ice-age glaciers formed only on its highest mountains, and glaciers carve sharp ridges and deep troughs rather than tabletops. Its even skyline comes from level rock layers, which is why the walls of the Grand Canyon look like a stack of straight bands.