Why can't peingins fly?

In one sense, it could be stated that penguins do fly. It's just that they fly underwater, rather than in the air.

There are a number of reasons why penguins do not fly in the air.

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, they don't fly because they don't need to.