Re: Why is RNA used as a primer to initiate DNA replication?

The short answer to your question is another question:
"Where would the DNA primer come from?"

RNA polymerases do not require a primer. They begin producing RNA from any appropriate start signal in the DNA genome (or RNA genome in the case of RNA viruses). Thus a cell can make new RNA molecules, including RNA molecles that can act as primers for DNA polymerases, without having a primer to begin with.

The fact that RNA polymerases do not need primers whereas DNA polymerases do need primers, is just one more of the dozens of lines of evidence that suggest that RNA preceded DNA as the molecule used for genetic information in the time period billions of years ago between the "primordial soup" of non-living matter and the fist replicating organisms that used DNA for genomes. By the time DNA and DNA polymerases came into existence (probably by evolving from RNA polymerase), RNA was already available, so the need for a primer was not prohibited and was perhaps even beneficial in some ways. One example of how it may be beneficial is in regulating DNA replication. To control when DNA can replicate, the organism could control when primers were produced by the RNA polymerase. The RNA organisms would have been evolving for over a billion years, and would already have evolved mechanisms for regulating the RNA genes and RNA polymerases.

The results of billions of years of evolution sometimes appear so clever or wonderful that it is difficult not to believe that all existing organisms today would use the "best" design possible. People tend to think of "survival of the fittest" when in fact evolution works more by "elimination of the weakest". Survivors only need to be fit enough to survive, not more fit than every other individual. The fact that all DNA polymerases known today require an RNA primer does not mean that a DNA polymerase that operates like an RNA polymerase without needing a primer is impossible or less fit. It only means that the organisms on earth today appear to have evolved from an organism that happened to have a DNA polymerase that required an RNA primer, and that the evolutionary jump to DNA polymerase that does not require a primer is for some reason so highly improbable that we have not yet observed that jump.

Your question about "Why have humans failed to evolve a DNA primer mechanism?" implies that you share the common misconception that humans are more "highly evolved" than other species. In reality, all species alive today are equally evolved, we are all equally old. Organisms that have aquired the ability to live as large multicellular animals are no "newer" than organisms which have aquired the ability to break down antibiotics produced by humans, for example. If you were searching for a DNA polymerase that did not require an RNA primer, you would be more likely to find one among the hundreds of millions of non-primate species on earth than among the dozens of primate species.