Game engines are pretty much completely sitting on complex tree structures. Way more than the DOM. The main reason the DOM all looks so tricky is it is a hack on a hack on a hack. A lot of game engines have been rewritten from scratch many times over.
Yes, trees are involved in games too, but not at the level I'm talking about.
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.
