One way of looking at it - imagine any real world massacre of native americans. Now imagine that instead of native americans, they are orcs. How much needed to change?
Lots of stuff about the native americans change - their religion, their visual apperance, their culture... they still remain a tribal culture with warriors. But the human's opinions about those orcs? their discussions about why it's okay to subjugate them and take over? none of that changes at all. Practically everything that humans say about orcs in a D&D campaign could have been a real world attitude about native americans.
D&D is constantly reinventing itself... it basically releases the same ideas over and over again, but with a contemporary coat of paint. So we have to ask ourselves, in the year 2020, what do we mean when we talk about a struggle between good and evil? And if the evil you're slaying is a dragon, demons, skeletons, vampires, whatever, then sure--we're good, they're evil, let's do the mash (it was a graveyard smash).
But if our campaign's antagonists are people (and orcs are people - unlike skeletons, they have a culture and religion and all that jazz) - we need a different story than "they are just born evil, and they have low intelligence scores, it's okay to kill them."
Like, many D&D campaigns are accidentally about "what if racism & genocide was good, actually? what if there was a world where genocide was moral? wouldn't it be fun?"