Because human beings are prone to gross excess and jealousy. And hunters are the worst of the lot. Hunters (as a whole, some particular examples come to mind though) are by far, the most jealous bunch of people I know. Which is why you can't ever tell someone where your duck spot is, or where you shot that big buck, or where you have been seeing turkey's strutting mid-morning when most are henned up.

Someone will always have to prove that they are better, or faster or killed more or bigger. This is how people justify their means and how they feel good about themselves. "I shot that thur dur at 600 yards with my thirteen-ought six! Yussir, took um rightoffahis foot. Musta been dat one Earl got a picea last yur..."