I don't know. Googling, I was able to come up with some examples, but I've never seen them in real life despite growing up in a farming community.

My speculation: you only harvest in dry weather (so traction differences don't matter), and harvesting goes faster than say tilling, so maybe tires are preferred. You'll also see big fleets of combines going down the road to the next field; I think tracked vehicles are frowned upon on public roads.