TLDR; Unova is not the entire continental United States. It would've been urbanized long before the Old West was a thing.

"Unova is the fifth main series region and is the setting of Pokémon Black and White and Pokémon Black and White Versions 2. Unlike the previous regions, which were based on different islands around Japan, Unova was based on an area that spans 2 American States, New York and New Jersey. This land is distant from all other currently known regions and thus has different Pokémon. Also because of this, the Route numbers restart at 1. For those who have not been playing the other games, this fact can be confusing. It has an incredibly varied landscape; with two mountain ranges, woodland areas, marshes and a desert spread across its islands. The map of Unova has nineteen major settlements, thirty-one places of interest and twenty-three routes marked on it." (source)

"The Unova region (Japanese: イッシュ地方 Isshu region) is a region of the Pokémon world. It is the setting of Pokémon Black and White and Pokémon Black 2 and White 2. It was the fifth core series region to be introduced.

Unova is far away from the four other large regions, and the Pokémon which inhabit Unova are diverse and different from those of Kanto, Johto, Hoenn, and Sinnoh. In his director's column, Junichi Masuda suggested that the geographical and social background for Unova draws inspiration from the New York City metropolitan area.[1][2] Although it is geographically based on New York City, it draws inspiration for its landmarks from the entire state of New York, which features a mixture of rural locations and urban areas as well as factory towns and industrial complexes." (source)