> When people can see all around them that gender, nationality, race, and religion convey useful information about the people they meet
> absolute denial of the obvious
You say that as if it's assumed, but I don't agree and neither do very many others. I agree it's obviously a human instinct to think that, but that's true of many other bad ideas - we're not beasts, our intellectual abilities take us far beyond our instincts. That tribal instinct has led to terrible destruction - arguably it's caused more harm than any other idea.
Of course I'm not speaking in absolutes (and I hope you don't you mean to). There's a little bit of information there, but generally I find people are the same everywhere; there are good and bad, smart and dumb, happy and sad, capable and incompetent, kind and cruel, etc. just as much in every group. It's really the individual that matters.
It's also essential to liberty and basic fairness: An individual need to be judged on their own merits, by their own action, and not by the color of their skin, their nationalities, their religion, gender, sexual preference, etc. If we say, "people in religion X are thieves", then it will be an injustice to almost all the individuals in that religion. It leads to widespread injustice even in peace, when a skilled woman or minority, for example, is judged instead on their race or gender and denied the job.