Here's why:

* Definition: Interior plains are vast, flat, or gently rolling areas of land located far from major mountain ranges.

* Erosion: They are often formed by the erosion and deposition of sediments over long periods, creating a relatively flat surface.

* Lack of tectonic activity: Interior plains tend to be located in areas of low tectonic activity, meaning there is less uplift and mountain building.

While interior plains can have some variation in elevation, their overall relief is significantly lower compared to mountainous regions.