I've always understood it as a sort of historical anchoring bias. Compared to the areas that were settled during colonial times, Ohio was indeed "West" in people's minds, just less so than the Pacific coast.
If you look at a map, it's clearly not. Not "mid", which would be from Chicago to Denver, and even more not "west", which would be west of Denver.
But "midwest" is a term for a region, and terms last longer than the reason for the term. When the term was coined, the US essentially ended at Chicago, and Ohio was midwest.
Hmm. I must be in a mellow mood today. I've made the exact same complaint you did when others said Pittsburgh was "midwest".
I've always understood it as a sort of historical anchoring bias. Compared to the areas that were settled during colonial times, Ohio was indeed "West" in people's minds, just less so than the Pacific coast.