why can't humans fly?

Easy. Horsepower to weight ratio... It takes horsepower to generate lift to support weight against the force of gravity. Humans don't have enough horsepower to do the job without a LOT of technology. A number of years ago, a human powered craft, the Gossamer Condor was flown across the English Channel, powered by a single man, a professional bike racer by the way, since he had the "legs" to do the job. Pun intended. Do a web search and you will find images and information. Throw enough horsepower at the problem, and the result is man walking on the moon. It is never actually made clear HOW it is that Superman "flies". It is simply ballistic in that he jumps? As per the "leaps tall buildings in a single bound" kind of thing? Or does he actually levitate in that he defies gravity in some manner? Or could it be something so simple as throwing yourself at the ground and missing?

to being, supermans ability to fly defies the law of physics. In terms of biology certain genes must be in place in order for evolution to select for the genes. Humans do not have these genes, like was mentioned in some of the other posts physically we are not structured and we don't have genes which nature can select for flight (lighter bone structure, bones growing homologous to bats wings, shorter bodies and legs, muscles that contain the right amount of fast twitch and slow twitch fibers to maintain flight)

Because we - humans don't have WINGS .. and any living thing without wings can't fly .

Note: Superman - is NOT human- He is a living thing from outer space who LOOK like a human.

Several different phylogenetic lines independently evolved with wings allowing flight. Our ancestors simply weren't among those lines. Instead, they evolved filling a biological niche that was on a different branch from those that eventually became flying animals. On that branch, the ability to fly was not important, and therefore not selected for.

Well, to fly you would need wings. Also, you would need to be able to generate enough speed to displace enough air to provide lift.

Off the top of my head:

Arms are not powerful enough.

We are also to heavy (birds have hollow bones).

We are not aerodynamic at all.

Feathers would probably help too.

Because we don't fully believe we can. Once we believe, anything is possible. Uh ya and gravity plays a part too.

you know what, the wright brothers asked the very same question. ask them.

not enough thrust... to much mass