I think on of the biggest influence of the race thing is "Theoretical Players" who are totally into tabletop game lore, story, and concept but don't actually play. People who treat it like a normal video game or book. Evil races work well from a gameplay perspective since your DM doesn't have to spend half the game establishing why your random encounters are bad guys, you can just go and attack the goblin and orcs because they're clearly evil and you need to fight something that can think and wield weapons. Nuance takes time. Its good if you're playing a game where you have that kind of time, like the people who don't actually play and just fantasize about it. Its annoying if your GM has to show the undead murdering innocent villagers before you can do anything about it because they might be the good ones and you don't want to screw it up.
Its like your cursor turning red in a video game. Kill these guys because they're bad and maybe get context later. The GM has an hour for this encounter and already spent ten minutes describing their location and equipment.
I think you hit the nail on the head. Evil races are a great shorthand for "this is an enemy", and they're considered Evil for a reason. If orcs and goblins in that world are widely known for invading peaceful lands, burning villages, raping half the survivors to death and enslaving the rest,
why would you give them a nuanced evaluation? They're not human, they don't
think like humans, they're not "misunderstood", they're not just doing that because of some great leader, they're simply fucking evil. If you see a band of orcs crouched in the bushes flanking a road, you know from bitter experience that they're up to no good. If someone chooses to believe that's some oblique reference to a real-life ethnic group, then I'd call
them racist unless the orcs speak in Ebonics and are called Tyrone and Shaqueefa.
It's very early in the morning and coffee isn't ready yet, but let me try to get some thoughts out here: I think this all comes from this tabula rasa idea many people have, that we're all exactly the same and only your choices affect who we are. Weirdly, a lot of the idiots spousing that idea also seem to believe that the only thing that's set about someone is their gender. Anyway, the point is that fantasy races aren't humans and they don't think like humans. Well-written orcs, goblins or kobolds would sound like sociopaths to us. They'd be like talking to a Neanderthal. They may be intelligent, but their mindsets are limited, explaining how they're often stuck living in primitive ways and rarely develop sophisticated societies, and in many settings they're
supernaturally bound to the gods that created them. Someone might argue these races are evil by Author Fiat and that's bad writing, and I would
disagree with that. Sometimes you just need uncontroversial bad guys, and whatever nuance may exist in their mindset is irrelevant to the story.
It's just instead of dehumanizing
humans to have our obviously evil guys (hi, Nazis. You're fucking overused), we go killing ugly monsters who
don't have loving families waiting for them at home.