First, "standards"-compliant HTML was never that important because the browsers were all so tolerant of malformed HTML anyway, and continue to be. And these days, the standards matter even less when there's really just Webkit and Blink. Gecko is a rounding error. You can write strict HTML to your heart's delight, or just kinda half-ass it. It doesn't really matter either way, since most users will use the same renderer anyway.

Then, even if you DO adhere to strict standards, they are so low level as to be useless. HTML + CSS defines a barebones, ancient layout system for simple documents -- which most of the web isn't anymore. Want to do a simple nav bar? There is no simple "nav bar" standard. You hack it by overloading a list and repositioning the components using CSS, then sprinkle in ARIA args on top of these overloaded components to tell the screen reader exactly how you decided to abuse the markup. Want to do a responsive popup modal with a scrollable table inside? You create your own recipe for tag soup, with your own media queries and wraps and scrollbar hacks and hidden elements. And every dev ends up doing this. At the level of abstraction of an actual UI, there isn't a standard, just different hacks on top of HTML. "Semantic" HTML and JSON-LD etc. all came and went. In the end you have every webpage reinventing the wheel to create the simplest of UIs, because there wasn't really a "standard", just super low level rendering primitives. There is no real system for componentization or templatization, and every server side language invented their own.

Enter JSX, which primarily gives you a more useful system of abstraction: that of components, composition, and inheritance, rather than meaningless tags like "div" or "span". This system allowed the creation of an ecosystem (as opposed to snippets) where different packages from different teams (or companies, or OSS) can interact by passing parameters/properties back and forth, something that was pretty much impossible to do when you were just overlaying HTML on itself (early jQuery libs tried to do that and ran into constant problems with clashing event handlers, z-axis issues, CSS namespace conflicts, etc.). From there you can create style and component systems (like MUI or Chakra), or roll your company's own, from which downstream app and UI designers can actual compose business applications. This was all just basic functionality offered by the likes of WinForms and Visual Studio back in the 90s or before, but which HTML never really had until JSX took over. Then from there we got an extensive ecosystem of powerful typing, transpilation (including to native and Electron), linting, etc., all built on top of simple syntax and somewhat reusable components. There's a very simple reason React took over and became the real defacto standard: It was useful. JSX outside React also sees adoption from other frameworks because it is useful.

Purity is meaningless and pointless. Whatever standards are set today will be reinvented and ignored and superseded by the next big company or movement. From Netscape to Microsoft to Meta, every major company overloaded HTML because it was (and remains) uselessly low level. CSS and JS themselves took years to be adopted and fought long wars against ActiveX, Flash, and even Canvas -- again because HTML was so limited.

JSX frees you from the underlying rendering concerns (which users don't really care about, and frankly most devs and managers don't either) in favor of UI components and useful business logic. Even if the resulting output can still be tag soup (if you don't use a good UI framework), at least it's consistent tag soup because of component reusability and inheritance. And in the best cases, it can generate much cleaner output than HTML because you start from strictly typed and strictly linted XML-like syntax instead of overloaded HTML tags and CSS overrides, giving you a degree of intra-codebase consistency that's very difficult to achieve, and inter-package interoperability that was straight on impossible in the past.

I'd argue that JSX and React are the best things to have happened to the Web since, oh, I don't know... the image map?