Here's why:

* Definition: Plains are vast, flat, and relatively low-lying areas of land.

* Formation: They are often formed by the deposition of sediment from rivers, glaciers, or wind.

* Comparison: They are significantly lower than mountains, plateaus, or hills.

However, there are exceptions. Some plains can have slightly higher elevations, especially if they are located on a continental plateau or in the foothills of a mountain range.