We have file tree viewers, and standard GUI components like Java's JTree. I only skimmed, but the article didn't seem to address the questions raised ("why?"),

We have file tree viewers, and standard GUI components like Java's JTree. I only skimmed, but the article didn't seem to address the questions raised ("why?"), but instead described the problem and proposed solutions. I think an answer to the question would be far more interesting and revealing... it might simply be that 1. it's hard; and 2. text is far better at hierarchical structures. Like his examples, lisp, programming languages in general, arithmetic algebra. So I think the hint is that the Human Processing System is adapted to handling hierarchical structures linguistically (i.e. symbolically: speech, writing, text). I suspect there is a very good and deep reason for this, to do with the kinds of operations (transformations) typically needed. But I don't know what that reason is.