I used to think that, but a few years ago an old opinion piece completely changed my mind on the subject. To summarize the piece’s argument (in case the site ever goes down), the key differences between superhero movies and westerns is that:
1. People go to see superhero movies because they like certain characters; people went to see westerns because they like westerns. To put it another way, if you want to see a western, the genre is broad enough that you can see all sorts of different movies. But if you want to see a superhero movie, you usually just want to watch Batman acting like Batman and doing Batman things, or Cap acting like Cap etc...
2. Westerns were small enough and cheap enough to make that directors and writers could experiment widely within the genre; modern superhero movies are so expensive that's there far more pressure to play it safe just so you’ll earn your money back.
3. Great characters usually only have a handful of truly interesting stories. A controversial point, but I think this gets at why superhero films tend to focus on either origin stories or constantly feel like retreads of the same ideas. 4. The actor is the draw of the western, while the character is the draw of the superhero film. With the western you can make different movies that emphasize different aspects of the actor’s persona or even have him play against type, while in a superhero movie the actor is something of an interchangeable widget that takes second place to the character. 5. At the end of the day, the audience doesn’t really want innovation or personal films all that much. This is only a crude summary of the piece’s arguments, so I really recommend reading the linked article above.
Superhero movies are the cowboy movies of our time.