It's true that any arbitrary file format's complexity doesn't necessarily have to be tied to the feature set of the authoring tool (e.g. LaTeX), but in this cas

It's true that any arbitrary file format's complexity doesn't necessarily have to be tied to the feature set of the authoring tool (e.g. LaTeX), but in this case much of the extraneous complexity of OOXML is tied to Word's feature set. In particular, the need for backward compatibility with features from previous versions of Word make this format more complex than it needs to be, given the current feature set.