Their XML-based format existed for a few years before it was made into an open standard, so the alternative would be a standard that didn’t describe the format of the zillions of documents ‘out there’.

Also, chances are nobody at Microsoft itself did know the exact effect of many of the more obscure flags.

So, you either get a clean start, or you get a standard that ‘all’ existing documents can be converted to without change/loss of formatting (‘all’ in quotes because there likely are truly old documents starting life, say, as WordPerfect documents converted into “WP compatible MS Word .doc”, edited a bit, converted to “MS Word .docx”, edited a bit more, converted into open office XML whose layout changed over time)

If you want the former, use https://en.m.wikipedia.org/wiki/OpenDocument.