The reason he's able to make this ask and not immediately be laughed out of the room is that Tesla's value is massively inflated based largely on the personality cult surrounding Musk himself. If Tesla starts being valued as a car company rather than a speculative tech company with a "genius" leader at the help (quotes doing a lot of work there) , the stock price will be a fraction of what it is now.
That's what shareholders stand to lose, and it's why Musk has any leverage to make this ridiculous ask in the first place.
Along the same lines, let's not forget L. Ron Hubbard, who founded Scientology some years after telling a sci-fi writers' convention that starting an actual cult would be a great way to make far more money off science fiction than just by writing it.
He's gotta pay back the Saudis for Twitter or else he'll be Khashoggi'd, so I imagine he's quite desperate to be asking this in the first place. I wonder how many people invested in Tesla as a way to raise funds for him personally, eg millionaire friends propping up the stock for him as a favor.