It has been a long time puzzle for everyone that why Penguin being a bird is unable to fly. After a detailed study, scientists have confirmed that a wing that is good for flying cannot also be good for diving and swimming and vice versa. Penguins are extensively found in Antartica and they depend on sea life creatures like krill, fish, squid for food to survive. So it is highly adapted for life in the water and spends about half of their lives on land and half in the oceans. This penguin's underwater adeptness may have cost it its ability to fly. There are several long-standing theories about why birds cannot fly. One theory states that some species became flightless because of a lack of predators on the ground. The other explains the 'bio-mechanical hypothesis - When the bird is flying and diving, it has to use its wings to do two different things. The bio-mechanical hypothesis is that you cannot build a wing that is good at doing both.
The only exception creature to this theory is Guillemot, a seabird which is good at flying as well as swimming and it looks almost similar to Penguin. When the amount of energy that the bird was using is analyzed, it has to spend much more energy while flying at an incredible speed to stay in the air.