Why do birds fly?
Because they can!
Because they have wings which makes it possible to fly... And to get away from humans because they're scared of us... And so they can't get hunted or attacked by animals.
Birds fly because they can, because they have wings. If I had wings, I'd fly too... who wouldn't?!?
For the same reason as you walk. Different means for the same purpose.
Because cats don't.
Fish gotta swim, birds gotta fly, other than that, I don't really know why.............;)
Because they have very delicate legs. Think about running on those delicate legs in a forest. I would think that escaping a predator would be much more harmful than letting it kill you.
The same way why we walk.