Not only is the standard overly complex, it’s also missing clarifications to the extent that it has to be considered underspecified. Moreover, MS Word in particular doesn’t precisely implement the OOXML standard (or any reasonable reading of it), but a buggy and subtly different variation of it.

Complexity alone would just make it laborious to implement, but the underspecification and subtle deviations of Microsoft’s implementation makes it virtually impossible to achieve full compatibility.