Why haven't humans evolved into super-humans, based on natural selection?

Natural Selection as coined by Charles Darwin refers to a
mechanism of evolution in which any genetic (inheritable) cause of an individual’s
phenotype (observable qualities) that would give the bearer a reproductive advantage
will over time become more and more common among the population.  Similarly, genetic
causes of impaired fertility will over time be weeded
out.

The concept embodied in your question is
the idea that, given enough time good traits will increase while negative traits will
disappear, eventually yielding superior or so called super
humans.

The problem with this idea is that the
human being as an organism is subject to the limitations of the design and structure of
the body.

For example, the strength of a human’s
arms is limited by the breaking point of the muscles and bones. Even with optimal
conditioning of muscles in a person who has inherited all of the traits for maximal
strength and development, the person could only lift the amount of weight bearable by
the mechanical structure of his body.  He would never be able to have super strength
such as that required to lift a tall building, as could
Superman.

Similarly, because of limitations of
the human design, humans could not evolve to
fly.

The concept of natural selection refers to
advantages in reproduction and avoidance of disease, but does not refer to the
acquisition of powers beyond the limits posed by the body’s inherent structure and
design.