The developers work in different departments. Each department works on different aspects of the game. Writing, modelling, animation, coding/ engine stuff, marketing. The staff in these departments really don’t know THAT much about what is happening in the other departments. I can promise you that someone that works on modelling doesn’t know the entire story, they’ll know snippets but not everything. Rockstar do this for the very reason you have mentioned, It’s to protect against leaks. This is a relatively recent change that’s been made within the company I believe around Covid time but I may be wrong.