Microsoft is just dominant and exporting its 40 year old legacy codebase as a spec. LibreOffice team is frustrated that the for-profit model is beating the OSS model and crying foul over mostly necessary complexity. If LibreOffice started from scratch they’d probably appreciate how much Microsoft serializes because a sufficiently complicated document saved to .docx basically provides a reference implementation.
We do need for-profit alternatives to Word, and I’m working on one in legal.
[edit: I hope to put some real thoughts on this down soon, but most of the wonkiness emanates from evolving functionality and varying trends in best practices over the decades. I’ve implemented a fair bit of the spec here: https://tritium.legal, but most of the hard part is providing for bidi language support, fonts, real-time editing and re-rendering, UI and annotations like spellchecking and grammar, not conforming to the markup spec. Spec conformance is just polish and testing. A performant modern word processor of any spec, however, is a technological achievement on the order of a web browser.]