"Why Marvel movies are so bad?"

People do want well written stories, deep characters and interesting backgrounds and Marvel does deliver those. So dunno wtf you're on about.

I don't think people watch Guardians of the Galaxy to see deep characters and plot, it's a comedy

Marvel has a decent array of movie "types." Rather than a single group of dark, gritty films it has a universe that has comedy (Guardians), pure action (Iron Man), everyman (Antman), mythical action/family drama (Thor), and moral/ethical dilemma (the Captain America movies). Marvel captures snapshots of the real universe, which has laughter and silliness right next to sadness and anger as human conditions, and built their franchise around them. There's something for everyone there, and if you come for the comedy you may stay for the drama (and vice versa). The world isn't simply grit and lamentations shot in gray and blue tones, which is what other franchises tend to offer. Marvel doesn't make bad movies, they make movies for everyone instead of just a narrow niche.

It's worth mentioning that Groot actually does have a killer backstory.

sounds like it's from some butthurt DC fan