OOXML is over 600 pages, yes. With lots of contradictions and filler words btw. And a "standard" (with scandals abound) that has most of the format as proprietary extensions. And MS Office/365 doesn't even keep to it, so other office suites never have as good support of the format as MS itself.

Use ODF to save your documents. MS started their "standard" because of it anyway, fearing losses in their customer base (that bound them to Windows).

That's doesn't seem unreasonable to me.
It's a hugely important format used by countless archived documents.
We can't just find out one day that nothing can read xlsx.

https://www.loc.gov/preservation/digital/formats/fdd/fdd000398.shtml

Depends on what you mean by "better". I have looked at the xml content before and never got the sense I could edit it anyhow, so any perceived benefit to it for me is far outweighed by the ginormous file sizes.