There's probably a less lofty, more mundane pop cultural answer for this: the generation that grew up on Saturday Morning superhero cartoons are now tastemakers, and the mainstreaming of nerd culture and fandom has incentivized studios towards making big budget live action versions of what used to be consigned only to cartoons on TV.

Also, while superhero movies are as old as Christopher Reeve's Superman films, or perhaps Republic serials, the mainstreaming of nerd culture and fandom has also created a public that's more open to elaborate world-building and lore in their summer blockbusters. So while superhero movies used to be more self-contained, now we have these sprawling multi-film sagas giving the studios an excuse to churn out multiple pictures a year.