Because they edited and published the HTML spec with lots of examples portraying HTML as an authoring language. If W3C actually does intend to publish further HTML specs based on WHATWG (which they haven't done since 2017), they should consider injecting their own navi/styles into the bikeshed-generated WHATWG material as part of their CMS setup (and they have tons of static HTML anyway).
W3C and WHATWG signed and MOU last year to work together on documents and specs.
Regardless, to define a language it only makes sense to author the specification from the language perspective. From what I've read, and honestly it's been many years since I have read their stuff, most of the authoring docs have a section in them talking about applications and editors creating the documents.
