We have space shuttles. We have hang gliders. We have planes, but without machines even the best human jumpers can only leave the ground for a few seconds. Why are we stuck on the ground while the birds fly high?
Most birds are built to fly and people really just aren't. Birds have wings, and people don't. Birds' wings are covered in feathers, which catch the air and force it downward when they flap. This pushes birds up in the sky. When birds pull their wings back up, they fold them so they don't catch the air as much. They then unfold them so they can push the air down again.
So why don't people just make fake wings? Birds are also much lighter than people. They have hollow bones. That means there's empty space inside of them and they weigh very little! People have heavy bones that keep us on the ground.
We also don't have the strength we would need to flap wings and make us fly. Birds do! They have special muscles that make their wings work. Their lungs are also designed so that when birds breathe they take in a lot of oxygen, which is needed to keep the muscles working over long periods of time.
Bats and many insects can also fly, and they each do so in their own unique ways. They have two main things in common with birds: wings and light weight. People have copied these basic ideas and made flying machines like planes, but because our own bodies aren't designed this way, we can fly on our own.
Remember, since we can't fly, it would be very dangerous to try. Stay happy and safe on the ground!