An obvious answer to this question is, “We cannot fly like birds because we do not have natural wings”. However, this answer does not explain this question fully because even if we attach artificial wings to our arms we cannot fly. Then how do the birds fly?
Birds, of course fly with the help of their wings. They have very light frames. Their bones are also light. Moreover, they have air sacs which make them even lighter.
Their muscles are quite strong in comparison to their body weight. They have a streamlined body which enables them to fly swiftly because air offers less resistance to streamlined objects. With the help of these muscles, they can easily fly with their wings.
When a bird flaps its wings, it pushes the air down with its wings and rises above. Our bodies are heavier than those of birds. As such we would need enormously big wings. Again we do not have any air sacs and our muscles are not comparatively that strong. Hence we cannot flap the artificial wings.
The birds with lighter bodies can fly more easily while the heavy birds face difficulties while flying. For example, the cock and peacock cannot fly very high. There are also birds like ostrich and penguin which are even heavier. They cannot fly at all. They can simply run. Thus we too cannot fly like birds even if we attach artificial wings to our arms.