I always felt this was covered. In both the books as well as the movies. Maybe not out rightly stated, but just common sense-wise.
When all of the creatures and living things of Middle Earth were created, the evil 'god(s)' felt life was there to serve them, that they were lessers rather than other beings. Certain 'gods' created Dwarves, or the Elves, etc. When the Orcs were created, I'm pretty sure that they were a perversion of Elf bloodline, bred to be barely human and only to serve.
Their role was never to think in the first place. Their role WAS to be cannon-fodder, as stipulated by not only the writing of the author, but the designs and machinations of the evil characters therein.
I always thought that was rather obvious. To see one sitting reading or acting peacefully is essentially a perversion of what they were intentionally designed for - aggression, bloodletting, warmongering, etc. An Orc would probably eat a book rather than read it, or even before contemplating what it could possibly be.
Just out of curiosity how would that apply to trolls? Did they have greater depth of thought than it seemed or were they just large predatory animals that could be coerced into service by more powerful (or more clever) beings>