Why can't penguins fly?

Answer: Their wings became short, stiff flippers for swimming, and their heavy bodies are built to dive, not to fly

They are simply too heavy; a lighter penguin could still take off — Weight is part of the story, but the wing itself is the problem: a penguin's flipper is short, rigid and narrow, with no flight feathers, and it cannot generate lift in air however light the bird. The smallest penguins weigh about a kilogram, less than many flying ducks, and they cannot fly either.

Their wings became short, stiff flippers for swimming, and their heavy bodies are built to dive, not to fly ✓ — Correct! Penguins descend from birds that flew. Over millions of years their wings shortened into stiff, narrow flippers that beat through water like oars, their bones grew dense instead of hollow, and their bodies became heavy, streamlined and well insulated, all of which make for superb diving. A flying wing needs a large, light surface and a light body; a swimming flipper needs the opposite. No bird has managed both, so as penguins became better divers, flight was lost.

Nothing hunts them on the ice, so they had no reason to keep flying — Penguins have plenty of enemies: leopard seals and orcas take them in the water, and skuas and giant petrels take eggs and chicks. Most species also live far from Antarctic ice, from Patagonia to the Galápagos. Flight was not given up for lack of danger; it was traded for diving ability, and penguins escape their predators by being faster underwater.