Why should people be immune?
HIV isn't traced back to an origin in one species of monkey. The direct ancestor of HIV is believed to be the SIV (simian immunodeficiency virus) in chimpanzees, which itself is believed to have arisen from a hybridisation event between two different viruses found in monkeys - the red-capped mangabey and greater spot-nosed monkey.
Many different African monkey species have their own type of SIV, all of them having evolved from a common ancestor to become adapted to different hosts, and one of which eventually became adapted to humans.
Now, as it happens, the human form of the virus doesn't work in at least some types of African monkey, and one explanation put forward is that the old-world monkey form of the protein TRIM5-alpha, which works as part of the immune system in primates, seems to prevent the virus from shedding its protective protein coat and reproducing.
Because, however, humans and old world monkeys have different forms of this protein, on account of them evolving seperately for millions of years, the human form doesn't work against HIV in the same way (if it did, the virus would never have become HIV, as it couldn't have infected humans). Note that this protein doesn't protect the monkeys against various forms of SIV - those versions of the virus adapted to infect old world monkeys.
So I don't understand your point. Why should humans be expected to possess the same form of a protein as cousins that our ancestors split from some thirty million years ago (give or take 5 million)?