If we have this incapability, what explains the abundant fiction in which nonhuman animals (both terrestrial and non) are capable of speech, and childhood anthropomorphization of animals?

That's not anthropomorphization.

Can you teach me to talk to the stray cat in my neighborhood?

Sorry, you're too old. Those childhood conversations you had with cats were real. You just started dismissing them as make-believe once your ability to doublethink was fully mature.

All of the really interesting stuff, from before you could doublethink at all, has been blocked out entirely by infantile amnesia.