Meh, I think there's nothing magical about the HTML syntax that makes it particularly great for expression DOM structures. Except familiarity of course.
1. HTML is a widely implemented standard. What you learn and know about HTML, and what you create with HTML is widely applicable. Not so much for your HTML alternatives, like Aberdeen.
2. HTML is what the browser accepts, which means you end up dealing with it anyway, just with a transformation in between, making things harder. The bigger the transformation the harder it is. To develop with Aberdeen you still need to know HTML, but you also need to know Aberdeen and how it transforms into HTML. And, you typically end up needing to learn how to mentally transform backwards as you debug, e.g., using the browser dev tools, looking at problem HTML, and deciding how to change the Aberdeen code to fix it.
1. I'd guess that of all the knowledge a web developer accumulates about HTML and CSS over the years, less than 1% concerns HTML's syntax. Everything else still applies.
2. As a client-side JavaScript developer, what you're actually dealing with is usually not HTML, but the DOM. JSX only resembles HTML rather superficially. It gets translated to a series of DOM method calls. HTML itself has no support for event handling, control structures, composition, etc.
That being said, going with pure JavaScript is of course a trade-off that also has downsides, such as indeed familiarity, and not being able to just paste a block of static HTML into your code. (Though there's a tool to help with that: https://aberdeenjs.org/Tutorial/#html-to-aberdeen)
