Why can't slugs fly? Why can't pigs fly? Why can't we breathe underwater as fish do? Why do you have blue eyes while I have brown? We have evolved to fill a niche. Birds have evolved to fill another niche. It's possible, though not certain, that we are the only creatures that look enviously at birds and wish we could emulate them. We are certainly the only creatures that come close to emulating them (by artificial means). Indeed, your question could be answered with the retort, "Humans do fly, look at planes, gliders, helicopters, microlights, hang gliders, parachutes and parascenders".

Humans are built to do the things that a large animal does on the ground. Flying animals are not so large. Not many are more than 6kg. It takes tremendous power and energy for such a heavy animal to fly. Even the giant soaring animals of the past probably never weighed nearly as much as a human. The closest we come to flight is gliding with special equipment, and to get off the ground when we glide we generally need to use machines.

Floating in bubbles is another matter; we can use balloons for that, but even in the balloons we walk about, just like on the ground.

If we kept the same size and weighed a lot less, say 6 to 10 kg, we still would need to change our shape and our build drastically if we were to be able to fly; we would need a wingspan far greater than the spread of our arms.  We would be fragile and many of the things that we can do easily now, we could not do as flying creatures.

Then we might ask each other: "Why must we fly? Why can't we be stronger so that we can use more tools and carry more weights and dig, and build aeroplanes...?"

Cheers,

Jon