Why are there so many superhero movies made in recent decades?

The simplest answer is this - in the 20th century, superheroes were a thing for kids and "nerdy people living in the basement", so a blockbuster movie with such a topic would not appeal to the wider audience when the world was run by jocks and the mainstream "cool thing" was sports, military, and so on.

Now the "nerdy kids" took over the world in the 21st century with the internet revolution and the mainstream thing is to be into superheroes, tech, and other "nerdy/geeky" stuff. So now these movies appeal to the wide audience - the kids, and the adults which were into comics while being young in the 20th century.

Superheroes are just very popular right now, so studios make more movies about them to get more money. In earlier times these movies didn't sell that many tickets, so it was not a financially good idea to make them.

I believe that Nolan's first Batman and first Iron Man were the movies that have taken the niche of "superheroes in spandex" and turned them into gritty, realistic, mature stories in the mainstream.

• Advances in CGI have made it easier to render the visual effects of comic books. In the past, it might have looked cheap and cheesy to render something like Cyclops's optical plasma with practical effects. For example, look at the movies Spawn (1997) and The Mummy Returns (2001), which had notoriously poor CGI, and would have looked much better if produced today. It is also likely the case that these new comic book movies are not simply creating their CGI effects from scratch, but reusing and leveraging work from previous movies to the next. This offers cost savings and quality benefits.
• The audiences who were children in the past are now adults. For instance, if an 8-year-old liked X-Men the animated series in the 90's, then they would now be in their 30's, and willing to spend their income to watch X-Men movies.
• Comic book movies are objectively profitable. For example, Avengers: Endgame yielded $2.4 billion profit.
• Hollywood studios are increasingly moving towards producing safe franchises that are already well-known, as opposed to independent films that have no established fame. Examples of safe, episodic film franchises include Harry Potter, Lord of the Rings, Star Wars, The Fast and the Furious, Jurassic Park, James Bond, Transformers, Pirates of the Caribbean, Marvel comics movies, DC comics movies, Terminator, and Toy Story.
• Similar to safe franchises, Hollywood also likes to make safe reboots / ports to film. For example, people are already familiar with the G.I. Joe brand, so make a G.I. Joe movie. People are already familiar with the Transformers brand, so make a Transformers movie. Other examples include the Star Trek reboot, the Ghostbusters remake, the Robocop remake, the Total Recall remake, the Aladdin remake, the Lion King remake, the Jungle Book remake, the Jumanji remake, the Pet Cemetery remake, the Power Rangers movie, the Nightmare On Elm Street remake, the Halloween remake, the Ninja Turtles reboot, the Winnie The Pooh port to film, the Where The Wild Things Are port to film, and so on.
• Movies with dazzling special effects seem to be out-grossing movies with very plain visuals. Premises like super heroes, transformers, wizards, and jedis offer an excuse to load-up on robust CGI visual effects; and therefore gross lots of money.