Biology SE has tags for both evolution:
Changes in the heritable attributes of populations of organisms over time. Major mechanisms include drift, natural selection, mutation, and gene flow (migration).
and human evolution:
The study of evolution with a particular focus on questions about the evolution of modern humans. Questions often incorporate the study of our nearest ancestors, both the extant primates and extinct pre- Homo sapiens.
Under the second tag, similar "why this and not that" questions have been answered, such as this one:
Why do we blink instead of winking each eye independently?
I'd say that these questions are not about human health per se, and therefore are off-topic. They are about a biological process (evolution) and belong to biology SE.