they found a winning formula so they go with it because it’s a way to play safe
The forces behind Hollywood are trying to get you annoyed with movies so you pick up a comic book where the dialogue is more serious. The reader has already bought in, no need to point out how ridiculous things are
It’s a current trend in film. We could argue that it’s the corporatization of film, but that was already the case in the 30s and 40s when MGM had more control over the movie market than Disney does today. Back then MGM pushed a lot of “GOLLY. THANK YOU, SIR” dialogue because they wanted the characters to be idealized images of the American people.
Studios wanted to expand their control over the process of films. Because TV production was more highly developped in this regard; tighter budgets and shorter deadlines than could be found in movies. So movie studios started hiring more TV labourers. That’s how Joss Whedon broke containment.
Another reason is irony poison not allowing people to commit to the bit and movies having to constantly lampshade that you’re watching a movie and it’s corny.