I.. uh.. don't know what to tell you. There is nothing not antiquated about it. It could, for instance, use a sane syntax for literally anything (I think only the raw math is acceptable; everything else is crazy). It could be written in a modern language that people can easily modify. Etc.
But TikZ in particular is more of a programming language than LaTeX is and really ought to be written in something closer to Python or JS.
TikZ is great in terms of the quality of the output, but I really don't want a "toolchain" for my site. Ideally the code for the graphics would be directly inline in a markdown file. I don't want a bunch of assets that I have to keep track of.
Plus, TikZ images render their TeX themselves, so you can't have KaTeX do it. This means you lose the benefits e.g. custom macros that you use everywhere, being able to change the math inline in the document if you rename a variable or something, or having all the math on the page scale the same way.
Of course, were I writing a book, I would use LaTeX. But I'm not. The needs for a casual website are very different. In particular I want fast iteration times, including on making edits to all of the graphics, which means that having them inline is much more ergonomic.