Nor a "first-class" TeX element either. \footnote is just a macro.
TeX itself is a set of typesetting primitives. LaTeX is a set of structurally-semantic, document-oriented macros built on TeX.
Some of those reasons are technical, though as noted elsewhere in this subthread, economic and other factors seem more dominant, and I severely doubt that LaTeX on its own will address the broader scope of issues raised in TFA.
LaTeX is almost never semantic. People are encountered to think only about the document presentation (mostly due to LaTeX's own failures).
As for footnotes being "first class". It's just a macro, nothing first-class about that when compared to HTML's solution.
https://www.pdfa.org/resource/iso-32000-pdf
The LaTeX project announced an a16y project in January of this year. A tool is now available, though success varies.
https://www.latex-project.org/publications/indexbytopic/pdf/
Of footnotes: LaTeX has the macro, it's common across multiple document types. HTML does not.
Various tools and packages that attempted to generate accessible pdfs from LaTeX have existed for aeons, all of they had a common characteristic: they sucked. I am convinced that by this point everyone who cares about accessibility has moved to other formats (like HTML).
"HTML does not"
<aside> is "first class".