[jplona]

https://en.wikipedia.org/wiki/Midwestern_United_States - The Background section has the explanation.

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.

[ikiris]

Wild. Learned something.

[fuzzfactor]

For a while there, the Mississippi river was the western extent of the USA.

In places like Ohio, you were midway there.