Except what if "characteristics within that structure" also have structure? Attributes are an ugly, crippled special case of a tree node; they make some sense in a markup language, but for serializing data, I see zero point in using them.

As for why UIs are represented in XML? It seems like a simple case of people getting used to it before JSON was popular. XML is a default choice for Java and for C#. It also looks vaguely familiar to HTML.