I truly believe this can be quite different between people. Personally I don't have "a" inner voice, but a quorum of three, all of which together form "my" thoughts

probably better than a bickering Italian couple! https://www.theguardian.com/science/2021/oct/25/the-last-gre...

That kind of variation isn’t too surprising to me. The claim that there isn’t one at all though I don’t really buy.

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.