The 21% for Indiana is a little deceptive, because the northern half of the state is not very wooded at all, but the further south from Indianapolis you get, the more forested it gets. You can see it on this satellite map.
The glaciers pushed all the land in the south upward and made it very hilly, which is good tree country. As for the north, it's not because forests were clear cut. It's actually for a worse reason. The north used to be part of one of the world's biggest wetlands, the Grand Kankakee Marsh. It's been almost entirely drained for farmland.