Do UIs need to be trees?

How do things change if we were to use a list instead? What becomes easier/harder?

Layout calculations might become slightly more expensive, though it seems that that may be acceptable since most time is spent rendering; and perhaps there are other ways to accelerate these calculations if needed.

The fact that most UIs are happy to provide a single linear tab order (and not something more involved that allows users to navigate the tree structure) makes me think that there might be something there.