Well, in many games like D&D, especially in previous editions, the idea is that alignment is this fairly absolute thing. If something is Good, then everything they do is Good (or neutral at worst). For example, back in 3x's Book of Exalted Deeds where they had good totally-not-poisons-honest, or in 5e when they had the Good god Moradin turn his back on the duergar because they failed their saving throws against mind control. Meanwhile (as I wrote on another thread) you have Evil orc gods who don't actually have that evil a portfolio. If you go by the original origin story, Gruumsh just wanted to give his people a place to live after all the human and demihuman gods claimed the good parts for themselves. But for that, the orcs were branded evil and everything they did was evil.
Logically speaking, orcs, elves, dwarfs, whatever, should be as alignmentally-diverse as humans, but because they have "Lawful Evil" or "Chaotic Good" in their statblock, people, including the game's writers come up with rationales like "orcs and goblinoids have less free will than humans do."
(I don't want to derail this anymore than necessary into an alignment thread, so feel free to ignore this rant and roll your eyes at me. But I kinda want to see a setting where, like, halflings and dwarfs got all the good land thanks to their gods and the humans and elves and whatever else got the shaft, and see what the PCs think and do about it.)