Why can’t they fly?
Well, you can’t blame the chickens. They started out flying, after all. At least their ancestors did. The ancestor of modern chickens, the wild red jungle fowl (also a darn good name for a rock band), wasn’t a great flier, but he could get around when he had to. The entire poultry family (chickens, turkeys, guineas, ducks) are adapted to living on the ground. Their beaks are better adapted to pecking off the ground, their feet to walking instead of perching, and their wings are smaller than other birds their size.
Enter us. We take a perfectly happy wild red jungle fowl and start selectively breeding to produce bigger pectoralis muscles (that’s the breast portion, for those of you who only see chickens in buckets) and eventually you get a bird who couldn’t get off the ground if they thought of it, which they don’t. Chickens comprise some of the thousands of artificial breeds that humans have created. Many domestic fowl, turkeys especially, have such large, um, breast portions, that they can no longer breed normally and must be artificially inseminated. This is a double whammy to the male poultry ego, such as it is.
For more wild facts about chickens, go to straightdope.com.