Why can't penguins fly?

Penguins cannot fly in the air because their bodies have been rebuilt for the ocean. Over deep time, useful swimming traits won out: short strong flippers, heavier bones, and a narrow shape that pushes through water. Those same features make lifting from the ground into thin air almost impossible.

The lovely twist is that a penguin's wings still work hard. Underwater, they flap for speed, steering, and chasing meals. Dense bones help them sink instead of bobbing up, and tight feathers keep cold water away from their skin while they hunt.