Some birds don't fly

Yes, some don't because their wings are not necessary or have been modified for a different purpose. Flightless cormorants in the Galapagos Islands have lost the ability to fly because for many generations they have not had any predators, and thus those without the ability to fly were not weeded out by predators. Penguins also cannot fly because they are specially adapted for swimming; the price of efficient swimming is the loss of flight. Similar to the flightless cormorants, penguins have few predators and seek out places of refuge away from most predators. Also large birds such as the ostrich are too heavy to fly and instead rely on their ability to run quickly to avoid danger.