Honestly, OOXML came out with Microsoft Office 2007 and according to Wikipedia the OOXML standard has not been updated since 2016 (“Version 4”). Open source competitors like LibreOffice had almost 2 decades to implement these 6,000 pages.
Although I have also heard the argument that MS Office doesn’t actually implement the “standard” OOXML format so that some inconsistencies in formatting cannot be avoided without reverse engineering. Not sure how true it is or how far from the standard it strays.