I disagree. Look up how modern material systems work. There absolutely are analogous things going on to CSS in PBR rendering, shaders, multipass compositing etc.
Is the whole CSS debacle a particularly unlikely concept to yield anything close to a reasonable performing implementation? Sure, but that is because nobody has applied the pressure necessary. People won't play a game that doesn't consistently meet a 30 ms deadline but they seem to happily put up with 3 seconds to final render and call it "amazing".
GPUs work here because games fundamentally operate on a list-like datastructure (the display list), with little interdependence.
Styling on the other hand operates on a tree like datastructure (the DOM), with lots of interdependence.
Cascading isn't something games need at all, and if it was, it could be precomputed. THis can't be done in webpages.
You don't have analogues for tree-structure-based "selectors" and anything more than straightforward property inheritance (e.g. inheritance of transformations in a skeleton) in game engines. CSS is much more complex in this aspect.
The DOM having lots of legacy issues is not the reason styling is slow.
CSS is designed for a lot of flexibility and can't be precompiled, which is why it is slow to style.