The lack of objectivity around this is absolutely astonishing from a community such as this. It exploded with complexity because frontend complexity exploded. Simple. Absolutely no people expected an "API call" from a frontend application 20 years ago, now people expect loading indicators on buttons (that make API calls) after pressing them. Just have a look at the comments on the recent Standard Notes / React Native…
In my experience, people don't start out with the goal of loading indicators on buttons. It's usually the mistaken idea that avoiding a page reload will make the action faster. Instead, you now have a slow Ajax call, and the button appears broken, so now it needs a loading indicator.
"a slow AJAX call"? What is even meant by that. Async JS is not slow. It's only limited by the user's connection speed and amount of data to be transferred and server response speed. This is a few dozen milliseconds but can be optimized down to sub 10 ms (the server and latency is the bottleneck not JS). A full page reload will of course take longer because instead of one tiny JSON object you have to send the full HTML back down the wire, and the screen also completely repaints and sets up a whole new DOM structure in memory for the same components.