If I would hazard a guess, I'd say it's possible that the region of the brain processing language has developed elsewhere than with your average human, leading

If I would hazard a guess, I'd say it's possible that the region of the brain processing language has developed elsewhere than with your average human, leading to less connections from language (Broca's region) directly to auditory region. This hypothesis would explain hearing the inner monologue when reading, as reading actually transcribes visual data directly to their phonetic counterparts.