I think it is a mistake to try to have this conversation generically about rpgs generally, because that is such a broad category to try to cover and the mythology of each game and what stories they are trying to tell are all different.
For the sake of why goblins are not just a playable character in pathfinder, but a core ancestry, all we have to look at is the mythology of Golarion and see that biological essentialism is something actively avoided with humanoid peoples on the material plane. On Golarion, it would absolutely feed into racist reductionism for any vast group of humanoids that have developed into multiple different cultures and kingdoms to be inherently tied to one alignment.
The gods of Golarion do not have the infinite power of creation that the gods of many other fantasy genre stories and games have. Many of them were mortals who obtained godhood. Those kinds of stories of entire groups of people created by the gods with metaphysical bonds to alignment are relegated to outsiders in the Golarion mythos. That some can break those bonds is tied to the in game reality that this is the age of lost omens, and the abilities of the gods to just speak reality into truth is limited and flawed in this game.
People arguing in favor of humanoid ancestries being alignment bound, in Golarion, are attempting to force the entire game into a single tiny box for their own comfort, without any attempt to understand the larger picture of the world that Golarion is. Dwarves don't only worship Torag in Golarion. In fact, it is perfectly reasonable to assume that there are dwarves in Golarion who have never heard of Torag and have no social or cultural connection to him. The same is true of Goblins and Lamashtu.
My problem with the entire debate around this issue is that it is way, way more tied to imposing limited perspective ideas on to the setting as a whole, often pulling those ideas in from other source material, making a convoluted mess of hypothetical situations that are not relevant to the game world. It is understandable for people to want to push back against people that are doing so, and ask them what motives they might have for doing so, especially when you hear phrases or language that gets used in other real world conversations, but the bottom line is that the whole hypothetical argument is essentially trolling when presented in the context of asking the question of why goblins are core ancestry in Golorian. The answer to that question is well known, clearly stated, and completely not a contradiction of any existing materials.
It is also understandable why someone might ask it if they have only read one AP or have had limited experience with paizo material, but there are plenty of places where folks asking the question earnestly can be directed to have that question answered at this point.