Why do some scholars combine the Plateau and Great Basin into an Intermontane culture area?

Scholars sometimes group the Plateau and Great Basin into an Intermontane culture area because the two regions share geographic and ecological continuities that make their boundaries difficult to define precisely.1

In the context of the culture area concept, the southern boundary of the Plateau ecosystem gradually merges into the northern reaches of the Great Basin. Topographically, the basin and range systems characteristic of the Great Basin transition into the high intermontane plateaus of the north. Because these regions are physically connected and share these transitional environmental features, anthropologists occasionally treat them as a single, broader cultural unit.12