Put simply, there's nothing noteworthy anymore about a character that has no flaws, no faults, and always does the right thing.
Quite the contrary: it's been decades since we've had a pure, unadulterated "good guy" hero on the screen. It's funny, the '60s and '70s embraced the antihero and the misanthrope, which gave rise to the popularity (in the comics) of characters like Wolverine, but then when he was brought to the screen he was de-fanged. Now you've got the Goddamned Batman hospitalizing everyone he meets, and babydaddy Superman spawning bastard superbabies who randomly drop pianos on people.
That's wrong with having a (super)hero one should aspire to emulate? That's the point! I'm surrounded by moral crises and questions. I don't need a superhero movie to see that.
And I sure as hell don't need a Superman or Captain America who go around pouting, doubting themselves and asking "why me?" and "what should I do?"
Screw that. I want John Wayne. I want Superman.
And another thing... I've heard this "Superman can't be interesting" tripe for years. He was certainly interesting for 70 years before anyone ever thought to make him "dark."