Mothers and teachers often say they need another pair of eyes on the backs of their heads. and another pair of hands would come in handy in many situations. you can imagine that these traits would have been advantageous to our early hunter-gatherer ancestors as well. according to sound evolutionary reasoning, what is the most likely explanation for why humans do not have these traits?

Because these variations have probably never appeared in a healthy human. As tetrapods, we are pretty much stuck with a four-limbed, two-eyed body plan; natural selection can only edit existing variations.

Explanation:

Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. This key mechanism of evolution causes  changes in the heritable traits characteristic of a population over generations.

This occurs partly because random mutations arise in the genome of an individual organism, and offspring can inherit such mutations (Variation).

Natural selection acts on the phenotype, the characteristics of the organism which actually interact with the environment, but the genetic (heritable) basis of any phenotype that gives that phenotype a reproductive advantage may become more common in a population.

Over time, Natural selection can result in speciation (the emergence of new species). In other words, natural selection is a key process in the evolution of a population.

Therefore, as tetrapods, we are pretty much stuck with a four-limbed, two-eyed body plan; natural selection can only edit existing variations.