This is all down to physics and evolution, and the engineering of human and bird anatomy.
Humans evolved from apes that lived in trees, and our bodies are adapted for living on the ground. We have strong, solid bones that support out dense, heavy muscles and the rest of our bodies. Birds have hollow bones, and have evolved feathers to help them fly. Their muscles have evolved to allow flight (although there are still some birds that do not fly any more, like emus, penguins and ostriches).
I found the following line on a website:
“Birds are the ultimate flying machines, though insects have been doing it for longer and in many cases are more versatile; their small size makes the physics of it much easier. Birds fly far better than any machinery mankind has yet made and like our machines they use their wings and tails in a variety of ways to achieve their expertise.”
Being more serious it’s question of evolution – birds evolved the ability to fly as it allowed them to escape predators and feed in areas where other animals can’t – we didn’t but have different abilities like those of running etc that allow us to do the same thing.
Evolution means that birds are specially adapted to flying with hollow bones, feathers and wings whilst we are better adapted to land environments having heavy strong bones and the ability to run etc.
Evolution means that if there is an environment where there is food an animal will evolve to fill that ‘ecological niche’