Why chicken can’t fly?
They can, especially when they are still chicks that have started flying. They cannot fly far when they are full grown because they are then much too heavy. Domesticated chickens are forced to eat feed loaded with proteins, grains and along with many other artificial supplements meant to alleviate or prevent diseases; stuff to fatten them fast for faster growth and faster slaughter.
Wild chickens without clipped wings can fly much greater and higher distances. Free range chickens get more exercise and need to have their wings clipped to prevent them from flying more than a few feet.