Because they have the power of using their feet almost like hands; as for carrying food to their mouths, scratching behind their heads, etc. When they walk on the ground, they tread not merely on the claws, like other birds, but on the whole of the foot. Their hookshaped upper mandible is articulated, very moveable, and serves the purpose of a third foot in climbing.
Because it has been said, but untruly, that they must always be kept in pairs, a single one not surviving the loss of its mate.
Because their wings are short, and unfitted for long flights. This is, at least, one cause. In the Philippines, for instance, many families of parrots are confined to particular islands, and never met with on others lying in the immediate vicinity. - Blumenbach.