This started because Zuckerberg added a "public-service clause", wherein he could go into public office for a period of 2 years and then come back to his role as CEO of Facebook. Adding this clause caused friction with members of the board, which proves it's important to him.

https://www.theverge.com/2016/12/8/13889308/mark-zuckerberg-...

https://www.vanityfair.com/news/2016/12/mark-zuckerbergs-pol...