BECAUSE IT IS A GREAT WAY TO LIVE!
A true higher conscious experience of life, so removed from our destructive stasis of defection from the mud (which is where we really emerged from dust to dust, and water to mud, and light to clay, time to movement and space to play.)
Imagine flying...first of all. Whoever invented that finally got it right. Ask the angels. Ask the flies.
Then imagine flying over the whole world, hugging the terrain yet keeping far enough away to simply be cradled by the whispers and long flows of the wind.
Then, on top of it all, you CAN land and walk around, pecking at juicy exotic bugs, feasting on the ripening of fruits, and you CAN also float and feel the sea bucking, picking up fresh sushi from the freshest and most remote spawns, lakes and tributaries.
The cream of this traveling existence from gooder to goodest comes with the finding of the old familiar neighborhood of the ancestors, finding the tree branch or nook to build a cozy nest, and it's hump, hump, lay an egg. Then it all starts over again!
Why do cranes migrate? Pffff, what a question!