OOXML has some serious problems, but "proprietary" isn't really one of them. OOXML is actually
two standards: OOXML Transitional and OOXML Strict. OOXML Strict is, by most accounts, a reasonable standard -- some bits open enough to interpretation that there will still be some problems between implementations, but HTML has the same issues.
OOXML Transitional, on the other hand, is filled with specified items that are just holdovers from MS's native implementation(s) of Office file formats. This is, more-or-less, the original submission from MS, and it was roundly rejected on its own. OOXML Strict solved enough of the problems that the standards bodies were willing to accept the dual standard -- OOXML Transitional used only for documents that are converted from old formats, and OOXML Strict used for new documents.
The problem with corporate-sponsored standards, though, is that the sponsoring organization usually has the first or leading implementation -- and the implementation itself becomes more of a de facto standard than the published standard. In the case of OOXML, Microsoft shipped OOXML-Transitional capable systems pretty quickly. But they still haven't shipped a product that reliably writes OOXML Strict documents (though Office 2013 does read such documents accurately).
Because the leading implementation is so... odd... the other "big players" haven't made a significant commitment to OOXML.