I never said it has anything to do with XML. OOXML is extremely complex for little reason. Even though it is also quite complex, ODT is much, much simpler.
There are actually reasons for some of OOXML's weirdness, just not good ones. For instance, it appears the reason why OOXML is pretty much the only XML-based document format which doesn't use a mixed content model is because there's a huge amount of prior art that'd have made it impossible to patent if they had. (Microsoft tried anyway though.)