Yep. I'm well versed with static site generators but every one I've ever worked with has been heavily template based rather than being an actual layout engine that map onto HTML+CSS+SVG. i.e. They all require you to still write in HTML+CSS+SVG rather than being a generalised way of writing HTML+CSS+SVG without dealing with the warts of those languages.
I think HTML+CSS is already a pretty good layout engine, so people don't really bother. In fact I think it's so good it's used even when it really shouldn't be, like with Electron applications.
I don't entirely get what you mean by "layout engine". A WYSIWYG editor perhaps? Or maybe a canvas like Microsoft OneNote, where you can draw and put text boxes? Or a word processor? Why isn't the usual Markdown (or the like) approach enough?
My gut feeling is that you cannot (fully) abstract away HTML/CSS if you want the result to feel like an actual website.
With Astro, MkDocs or docfx, I do not have to touch HTML, except maybe for creating the master layout and/or transformation rules, if needed.