IMO it's mainly Google and Chrome/ChromeOS trying desperately to keep people away from the competing Windows and Apple ecosystems, which requires that the browser do essentially anything a stand-alone program could do. This doesn't explain why there are a million JS frameworks, but it does explain why the browser has ballooned in features and complexity over the years.

>This doesn't explain why there are a million JS frameworks

Not sure if this is a valid answer, but I would assume that both Microsoft and Apple provided decent and easy to use frameworks to build applications on their operating system. This setup a particular amount of vendor lock in, the library is already there and doesn't need downloaded/installed making it easy on the end user.

On the other hand, frameworks were not provided by the browser manufactures initially. Also, everything is downloaded so 'include foo.js' is just as easy as 'include bar.js' opening up a vast ecosystem of frameworks.

The first sentence is a valid reason:

> Because the browser nowadays is an "operating system" that needs to be capable of handling all manner of dynamic software: maps, video, image editors, games, etc.

If frontends didn't use the "OS" capabilities of the browser, how would one build a multi-user real-time text editing app like Google Docs?

Edit (and off topic): Writing this comment gave me flashbacks of using Mapquest online for driving directions before smartphones. You had to print the directions out and bring them to your car on a piece of paper. It worked though!