Filmmaking works best as a collaborative medium.

When one guy is in charge of the film and everybody working with them is so in awe of them (or financially beholden to them) that they’re afraid to say no, afraid to tell him his ideas are dumb … then his dumb ideas will make it into the film and make it worse.

You also often see it with really big-name directors. Their early films are amazing, but once they have their huge reputation and people are afraid to push back on their bad ideas, their later films can end up being bad, or at least weird.