Because the tools to create it are relatively new, and the sites you're speaking of had already been written. What high-profile site didn't already have a massive legacy React/Angular/Vue codebase, and a team of framework-trained developers, as of 2021?
TBH that's not a framework vs HTML issue, that's just sloppy or inexperienced devs
If the rendering framework doesn't support accessibility as a first class citizen (or better yet, automatically creates/makes accessibility part of what is rendered), then the framework is not suitable for production use.
> entirely on you
of course - choosing a framework or hand rolling one makes no difference. It's still not production ready, if it doesn't have accessibility built in.
Choosing, for example, whether to use a div with a click handler, or a proper button, is a decision you need to make regardless of whether or not you're using a framework.
So it seems incorrect to say that you lost accessibility in this case due to the framework. You lost accessibility due to developers who didn't understand the web platform well enough to design proper accessible controls. If they hadn't used a framework, they'd probably still not have made the site accessible.
And i actually think that it is _better_ for a developer not to have to do their own accessibility controls, but delegate this problem to the component framework. They have to know about the problem, but not have to spend cognitive budget thinking about it while they compose their UI from framework components - this should _automatically_ be taken care of by the design of the framework.
Frameworks also have linters that will help a lot, it’s on you to use them
One semi-objective thing we lost was accessibility. Much of the site is impossible to navigate via keyboard due to naively re-implemented behavior like links being divs with click event listeners. It's actually somewhat worrying - when regulations hit us, we'll have to scramble hard to get back what we threw out.
But all in all I kind of agree with you that it's very hard to find high profile examples of sites that are "HTML first". I believe in it, but haven't actually seen it pan out. But I suspect the reasons for it not panning out might be purely in education. By the time HTML became powerful, frontend dev education was already deeply framework-focused.