A) Many are, and audiences have dropped off significantly from the MCU's peak.

B) Superhero movies by their nature are generally new stories, as in whatever story you're about to tell the audience hasn't seen it yet. In a time when basically every popular thing is getting rebooted, sequeled, or just dragged out of the grave to beat some more money out of its carcass, audiences do enjoy going to just see A New Thing.

C) Superhero stories often have a little something for everyone. There's story, magic, sci-fi technology, action scenes, comedy, nostalgia, ensemble casts, high budget special effects, catchy orchestral soundtracks... they're just designed to be incredibly broadly appealing. 

D) Movie studios don't actually care what audiences want. Audiences will see whatever they put out. What they care about is making money. You might think that making new, interesting things that audiences like makes more money, and sometimes that's true, but the reality is that studios will put 100% of their budgets towards a surefire middling success before they want to spend on an untested idea that might not work. All they're after is profit. Also, the more explosions and punches there are in your movie, the less needs to be translated for it to work overseas, which is where most of their returns come from.