Why does science call RH- blood RH-, as the Rhesus monkey isn't related to those without the gene at all?

"Rhesus factor got its name from experiments that Alexander S. Weiner and Karl Landsteiner conducted in 1937. These experiments involved rabbits that produced an antigen present in many humans' red blood cells when the rabbits were injected with the red blood cells of the Rhesus monkey. This led to the understanding of the genetics of Rhesus factor."

People who are Rh-negative still have the gene;

it is just a version (an allele) that doesn't produce the Rh antigen.

There are numerous version of that and many other genes.

When someone refers to not having "that gene", what they don't have is "that allele"!

There is a very tiny difference between the Rh-positive allele and the Rh-negative allele;

but the difference is enough to render the one allele function-less.