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.