Well first of all there’s only been about three that weren’t already grim’n’gritty comic books to begin with.
Secondly, speaking of said comics, they tend to come from the Frank Miller school of writing that everything has to be this depressing slog that tries to sound smarter than it or it’s writer have ever been (V for Vendetta, Watchmen, ect).
Thirdly, the ones that don’t go completely miserable in tone (Kick Ass, Deadpool) only get their R-Ratings by hyper-inflating the gore, sex and coarse language, because apparently something can be adults only if you say “Fuck” a hundred times.
Fourth, and this ties into the above, these tropes are trotted out by lazy writers in the first place. People who believe that “Mature” means to be either unbearably cynical or to hyper-inflate juvenile material. And nine times out of ten the writers who pull these stunts can’t write a decent story to begin with.
Fifth, the average Pixar or Captain America movie is more intellectually mature than these lifeless husks could ever hope to be.
Fact: There is currently no Marvel or DC Superhero that absolutely NEEDS their film to be Rated “R”. If any Marvel or DC Superhero movie comes out with an R-Rating, then that means it’s content has either overdone it on sex and violence, or the writer has been listening to “CRAWLING IN MY SKIIIIIIIN” during the entire production.