Perhaps because they still believe in the promise of Web 1.0, where their app is a graceful-enhancement over an initial server-rendered document, that can be easily worked with at a DOM level by any HTML scraper, easily baked to a PDF and printed, easily re-laid-out for better readability just by changing the browser font size, easily text-to-speech'ed (including ARIA roles, alt text, etc), easily re-styled with a user-agent stylesheet, easily intermediated by browser extensions, and so forth.
I've yet to see a WASM-driven web application that's any less opaque to these technologies than a Flash or ActiveX applet would be.
Also mentioned Web 2.0 for a reason. The "cram full-blown app into browsers" web, that uses Rube Goldberg machines behind the scenes. Joking aside, these (btw, also opaque) dynamic applications can hugely benefit from a new paradigm. While the Web itself can go back more to its original 1.0 roots of hypermedia, and allowing more and simpler browsers to wield its content.
It's nearly ten years old now but I remember being absolutely blown away by React Canvas, a UI toolkit that leveraged React but instead of rendering DOM nodes it rendered to a <canvas/> tag. Beautiful, 60fps stuff. All written in JS. Unfortunately all the demos seem to have disappeared since but here's a blog post about it:
https://engineering.flipboard.com/2015/02/mobile-web
Point is, both Makepad and React Canvas have something in common: they ditched the DOM. There are both advantages and disadvantages to doing so but the relevant point is that you don't need to use WebAssembly to do it.
I strongly recommend NOT doing this. It sounds like a fantastic idea in theory (who wouldn't want pixel-perfect control over all client viewports?), but there are so many caveats and edge cases that make it a nightmare in practice. You don't even have to get into accessibility or internationalization stuff to find the kinds of sharp edges that scared me away. Simple things like HiDPI, resizing viewports and drawing text.
I spent a solid 3-4 months of time on this path. I was so tired of fighting platform/browser quirks that it made sense. Now, I accept those quirks as battle-tested features and don't try to fight them anymore.
The only reason you should actually ditch the DOM is if you are doing something like Overwatch in the browser. Even then, I'd argue you would be a complete dumbass if you skip out on the power of CSS, etc. for purposes of handling your Menu/HUD elements.
Get prepared to be blown away by Makepad [0]. I have no affiliation with them, but just watched their most recent conference presentation [1]. The slides were made with Makepad itself and included, embedded, a full-blown IDE, a synthesizer app, a Mandelbrod to zoom in endlessly, and more. All running at 120fps. The presentation is for the most part live-coding with this setup.
What they want to do is bring coders and designers closer together, and while some code is in Rust they developed a DSL for the GUI parts that is close to how Figma works. These GUI's can run anywhere.
And I couldn't help thinking "Why would people have complicated stacks to create Web 2.0 apps for the Google Web, when they have this?", in other words an opportunity to break out of the browser straitjacket.
Btw. WebAssembly/WebGL isn't the only way in which Makepad is available. And while running well in the browser for a time, there were issues to be solved here (addressed in the presentation).
[0] https://github.com/makepad/makepad
[1] https://www.youtube.com/watch?v=rC4FCS-oMpg