Replying to my own comment, but if I had to guess (on absolutely zero context and evidence :p), I would say he wants to focus on crypto/Square, and Twitter is more of a time sink. Twitter is in a good enough state at the moment that the team can move without him so it's a good moment to leave.
Other than Musk for Tesla and SpaceX, I can't think of any of the major companies who would really be impacted much by their CEO leaving. Maybe Zuck leaving FB might make a small difference but IG seems decently independent of him anyway.
Zuckerberg by all accounts bullied Kevin Systrom out of the company and now it prompts you to sign in when trying to browse photos; doesn't seem independant
