Why can't penguins fly?

Penguins descend from flying seabird ancestors, but over millions of years they became specialised for chasing fish underwater. Their wings became short, stiff and flat, perfect for 'flying' through water, which is about 800 times denser than air. They also developed solid, heavy bones that reduce buoyancy, unlike the hollow bones of flying birds. Studies of diving seabirds suggest a wing cannot be efficient both for flying in air and swimming in water, so penguins specialised completely. The emperor penguin can dive deeper than 500 metres and stay underwater for over 20 minutes.