Because they fear smart people. Here's what goes through the mind of 'Bob the Average Meathead' when he see's someone demonstrate intellectual ability above the level of a 5-year-old: "Bob no like smart kid. Smart kid scare Bob! Bob must hit smart kid. If Bob hit smart kid, smart kid shut up and no one notice Bob is dumb. Bob hit smart kid." Repeat from Playground to Boardroom. By the way, the chicken recovered (unlike the chicken that you ate recently).
at 1:35 you can see his fat hanging out
Chickens totaly zone out if you do that too. Even after you stop petting their tummy they'll just lay there peaceful as can be looking half asleep.