Even on the smallest of scales, it requires a director to organize the efforts of several dozens of people to work towards a singular goal. Not only does a director have to make every choice with this final goal in mind for months, even if no one else sees it, they also have to maintain their sanity during the shoot.
All of this while also maintaining budget constraints, trying to please the studio, and working in exceedingly stressful circumstances. So it's no real surprise that Hollywood history is chock-full of horror stories where directors didn't make it all the way through production. In fact, it's kind of a miracle that anyone can make it all the way through.
There have been several high profile cases of mid-film director switches as of late, but this phenomenon is far from new. Some of the most beloved films of all time have had switched directors midway through production and still become classics.
These are the most notorious cases in Hollywood history in which a director left the film or was fired midway through production.