Why is this? Why aren’t more people
shifting to htmx  or related “simpler” libraries?
People in tech are generally terrified of saying “this seems too complicated” because it sounds close to “I’m not smart.” Combine with the snow-job culture of tech sales and, well, you get what we’ve ended up with.
In general the issue with small frameworks is that you need 5 of them then still implement a lot yourself. Vs a big bulky framework which takes time to really learn, but has integrated support for all the features you need.
It’s why many people use big game engines instead of making games from scratch. If you make a game from scratch you need to implement asset management, rendering, animation, collision detection, AI, and export to multiple platforms. Game engines have built-in support for all of these things.
And i do know that many people use static sites or simple blogging platforms. Most sites i visit are actually pretty simple and look like they were built on older technology. On the other hand, Facebook, Twitter, YouTube, etc. kind of need to use complex frameworks because they’re complicated apps.
validation is also trivial: you do can do it server side as was normal for many years. htmx also allows you to plug in to the HTML5 client-side validation API if you'd like: https://htmx.org/docs/#validation
there is no need for a complex front end framework for the vast majority of web applications. something like twitter or gmail could easily be built using a hypermedia-oriented approach, but when they were built hypermedia hadn't been advanced in nearly two decades. now it has.
Tools also have network effects. If you choose something like React, and you run into a problem, well, it's likely solved with a simple Google or Stack Overflow search. If you run into a problem with htmx or others, the community and support just may not be enough to solve the problem, and you'll have to spend time figuring things out yourself, time that could be better spent trying to solve your problem.
As to why the OP you referenced went overboard, that has more to do with the old adage of ‘life is not complicated, people are’. I agree with you, this stuff is not that complicated, it’s the people designing it that are.
If you think about how mundane making web apps can actually become, it’s no surprise developer histrionics found a way of creating amalgamations.
I count this as very popular.
While the tendency to over engineer web apps is strong, many teams opt for faster development.
 if you measure it in number of technologies / toolsets used.
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