Why can’t birds like penguins fly? Is it just because they don’t need to?

Question: Why can't birds like penguins fly? Is it just because they don't need to?
- Keywords:
  bird,
  evolution,
  flight,
  flightless,
  fly,
  penguin
Asked by marialovesmusic to Charlotte, Jo, Kevin, Louise, Valeria on 15 Jun 2012.
- Kevin Mahon answered on 15 Jun 2012:
  
  Flightless birds, like the penguin and the ostrich, have evolved from ancestors that could fly (hence the wings!). The reason is the one you gave – they didn’t need to. Over generations, they evolved a lifestyle that did not necessetate the ability to fly, hence why they can not!
  
  If there was a species that needed to fly to survive but could not – it wouldn’t obviously!
- Joanna Cruden answered on 15 Jun 2012:
  
  While penguins cannot fly in the air, when they are in water their wings help them to swim and fly through the water, ostriches and emus use their wings to help them cool in hot weather, balance and to frighten predators by making themselves look bigger

Question: Why can't birds like penguins fly? Is it just because they don't need to?