There is a lot more unusable arid land in the mountain west than there are warehouse rooftops. Adding bulk commodity ground mount PV to some portions of eastern WA where there is neither ranchland/grazing or irrigrated crop would go pretty far, in addition to the already existing massive hydroelectric dams.
Look at the already government owned land around the Idaho National Lab in south/eastern Idaho for similar.
Massive scale ground mount PV on otherwise unusable already government-controlled land is less costly and faster to deploy than putting things on roofs.
People who live in the eastern or central US states and have never spent a lot of time driving around rural areas of the "Mountain west" may also not fully appreciate how much land is not owned by any one specific person or corporation, but is already owned/controlled by either the BLM or by an individual state such as the WA Department of Natural Resources. This is very alien to how rural land ownership exists in eastern US states.