Because the tractor tires are filled with fluid for weight and traction.
It's like the joke i heard. Two guys were trying to lead their mule into the barn, but his ears would touch the top of the door and he would back out. The two guys got togeather and decided to raise the barn up a few inches. A neighbor came by and asked what they were doing the two guys told the neighbor about the mules ears touching. After hearing why, the neighbor said why not dig a small trench and lead the mule in. The two guys laughed and told their neighbor how dumb he was, saying it's his ears that are to long not his legs. Same moral.