There are a number of reasons why penguins do not fly.
Penguins eat fish. The fish they eat swim fairly deep under water, so flying is not an advantage in getting the fish, but swimming is. Therefore, penguins swim rather than fly. Quite simply, Penguins don?t need the ability to fly.
Penguins have very small wings, yet are very heavy & fat. The heavier bodies of penguins are the result of their special bone structure and the blubber that they carry. The blubber protects penguins from the cold. Cold water can remove heat from the body much faster than air, so a good insulator is required. Penguins are simply unable to create enough energy to take off.
If penguins can?t fly because they have such small wings, then why do they have small wings in the first place? The answer is that small wings are much more useful underwater because they have lower drag and greater agility. Their wing bones are fused straight, rather than angled like a flying bird?s, and this has the effect of making the wing rigid and powerful, like a flipper. The small wings and a streamlined body shape are ideal for diving in water. Larger wings would create too much drag, while the small penguin wings are excellent for high speed swimming.
Penguins also have higher levels of myoglobin and feathers optimized for the aquatic environment. Myoglobin is the main way penguins store oxygen during their long dives. The muscles of flying birds are filled with mitochondria and enzymes to power flight, and there is no space left-over for myoglobin. So, flying birds cannot spend us much time underwater as penguins because they have less myoglobin (less oxygen per body mass) and lower body mass (less over oxygen). Another benefit is that penguins are able to optimize their feathers for the aquatic environment. Penguin feathers are short and tight to keep water away from the skin and to create a smooth surface to lower drag. Flying birds have very different feathers that are fluffy to trap air for insulation. Wheeeeeewe !
Consider the following: Flight is the process by which an object moves through an atmosphere...by generating aerodynamic lift, propulsive thrust...aerostatically using buoyancy...without direct support by any surface.
So: Penguins do "fly"--but their atmosphere is water.