It's arbitrary. We generally refer to "West" as 'West of the Mississippi River'. Mid-West as Ohio-ish to around the Mississippi River (i.e. Midwest (Ohio, Indiana); Upper Midwest (Minnesota, Michigan, Wisconsin, Illinois)).

A lot of us still refer to Texas; Iowa; Nebraska; Missouri; etc... as just 'mid-west'. California, Oregon as 'West Coast'; Washington as 'Upper/Northwest Coast'; Arizona/New Mexico as Southwest; and Wyoming/Nevada as West.

Goes back to the days of expansionism in the early to mid-1800's, Texas and the Mississippi River were the "East/West" dividing line. Texas, Wyoming, New Mexico, Arizona, and Oklahoma also have the standard 'west' cartoon and movie stereotype (cowboys; saloons; tumbleweed; desert; etc...). In the mid-west it's more field or forest. And freezing cold wind. Lots of freezing cold wind.