For example, a "row" is not just a "<div>" tag. Its a div which horizontally fills its container. Centering contents with a "center" style attribute abstracts flex-box, browser compatibility, version compatibility, and the cascading behavior of CSS.
You move the incidental complexity of the web platform into the compiler which will always do the right thing. And in exchange you get the option to compile to a native or mobile app for "free".
But there are categories of application where that is not acceptable. The presentation is a tightly controlled aspect of the application's functionality. If you're designing an application with leptos or sycamore my suspicion is you would fall into the latter category rather than the former.
Also the ecosystem is really not there for XHTML, it never really took off. In practice it is close enough to HTML that it probably mostly works, but you are going to have problems.
The advantage is also very small, your emitter is simpler (you don't have to special case void elements and whatnot) and if you need to consume your own pages the parser is simpler. But that isn't worth much for most people.
It does make me sad, because parsing and even emitting HTML is a nightmare. But it won, so at the end of the day I find it easier to just accept that.