A forest cover map, for reference, so no one has to guess:
Globalforestwatch.com
Illinois is, indeed, mostly cleared, as is much of western Ohio. It's also pretty clear that Michigan, Wisconsin and Minnesota have northern lands that remain pretty heavily forested. I know in Michigan, at least, it'ss because much of the state consists of soils and climate that don't lend themselves to intensive agricultural usage relative to the areas south and west. We've got a very heavily agricultural area based off Saginaw Bay that travels southwest to around metropolitan Lansing, and a very narrow (but long) fruit belt along the coast of Lake Michigan. But much of the rest of the land used for agriculture is more scattered.