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.