This is conjecture, but why couldn't they hire the writing equivalent of a voice actor?

Honestly I don't think it would take much. Doing it ethically would take a little more money, but not much, for them.

Hire a prolific author with the writing equivalent of the "midwestern accent". I have a terrible writing accent, so it couldn't be me, but these people are out there. Pay them a bunch of dollars to ingest their entire corpus, and to produce more as needed. Use an LLM to match the writing style of this author during the post-training RL, and get a brand new Claude voice out of it.