It's not that I agree with the characterization in the OP, that these formats are deliberately obtuse. It's that I do agree about them being obtuse, and that being able to say that you can auto-generate bindings for them doesn't actually help to make them not obtuse.

I do also think that Office should have created separate formats for project files and export files; if an RTF can hold onto all the formatting details of a typical Word document sufficient for pixel-accurately rendering it for example, then they should have conveyed that better and promoted it as the default export format (along with the idea of an export format), rather than immediately hitting people with a popup that claims their data will be partially lost. If this does exist (just not as an RTF), this point still stands - I don't use it, nobody I know uses it, so it may as well not exist.

Current state of affairs is people passing around docx, xlsx, etc. files, which are project files, hence why they (have to) contain (fancifully) serialized application state. Imagine if people passed around PSDs rather than PNGs. Or if people passed around FLPs rather than WAVs, FLACs or MP3s. It's this separation between the features of a document / spreadsheet / presentation and the features of the authoring software that appears to be completely absent from Microsoft Office, and this is something that just based on the information I have available, MS can legitimately be faulted for. Transitioning from a bespoke binary format to an XML based format with schemas available did basically nothing to help this.

And while it might seem like that I'm suggesting that export formats are this cleanly definable, self-evident things, I don't actually mean to suggest so either. It'd have had to have been a business decision. To where to draw the line would have been a decision that apparently never came to be debated internally, from what anyone can currently tell in retrospect at least, from the outside.