It wasn't just Elon who turned that down, it was put to a vote and "More than 86 million shares voted against the proposal at a shareholder meeting in June, while fewer than 17 million voted in favor, Tesla said."
If Musk had come out in favor of the proposal and said: “Yes, it would actually be a good idea to hold Tesla to a higher standard of corporate governance and have someone take on part of my workload, so I’m not going to vote for myself” — do you think he’d be chairman against his will?