The 1e MM is supposed to be used to challenge and fight PCs, so of course they will be written that way. That's their intended function within the game. They are not some sort of dissertation on the societies of said monsters outside of that context.
And yet the trend continued in 2e, which included Habitat/Society and Ecology sections. And in Dragon magazines, which had articles (e.g., Half-Orcs, in #62, and The Humanoids, in #63, both by Roger Moore for 1e; despite the title, Half-Orcs is more about orcs than half-orcs) where they had
plenty of room to go into their society--and they used that room to talk about how awful they are.
If they had wanted the humanoids to be not always evil, I'm they would have thought to write that. You don't need to write a dissertation on it. You need one sentence, maybe two. In modern parlance, all you need is a hash tag. After all, the 1e MM included a fair number of good and neutral monsters who
weren't supposed to be used to challenge and fight PCs.
In the Half-Orcs article above, Moore talks about how even half-orcs raised by humans are only occasionally going to be neutral or, more rarely, good, but will be "unnaturally" (by human standards) Lawful, because back in 1e, orcs were LE, not CE.
Yep. Doesn't apply to arguments that say, "Why aren't there any exceptions in individuals?" or "Why is it that X monster CANNOT be a different alignment?" The exceptions in those cases neatly counter the arguments being used.
Yes, it does apply to those arguments, very much so. Because no matter what
you, as a DM, may decide, the books themselves show that the game expected orcs, goblins, etc., to be entirely evil, with
maybe an exception. And "this orc is OK, he's a
good orc," is
not a neat counter at all.
And anyway, even if and when the MMs blatantly say "not all whatevers need to be this alignment," that only actually
means something if the books actually
show this to be true, and not just as that one exception. Heck, skipping editions, Obould Many-Arrows: uniter of his peoples, broker of peace treaties, willing to put his own anger aside to form important alliances.
Chaotic Evil. Not even Lawful! In 3x, when orcs were only
"often" Chaotic Evil, they wouldn't change this one orc's alignment to even Lawful Evil.
That is why saying that the exceptions counter the arguments.