The US started as 13 colonies/states on the East coast. In the terminology, everything past that is "The West", and this general area is the middle-ish part of that, so "Midwest".
It’s the name of the region. The Great Plains aren’t particularly great either, they’re just big. It’s like how the Mediterranean isn’t really in the middle of the world
"Great" in that sense doesn't mean "good," it means big. You see the same use in a lot of bird names as in the great blue heron or the great auk, just off the top of my head.
Maybe on a purely east west dichotomy, but if we're using the typical 4 regions of the u.s. : Northeast, south, Midwest and west, then that is not right.