Microsoft produced the boondoggle 'Office Open XML' formwat in reaction to the moves made by several large organisations to standardise on the Open Document format as used by OpenOffice et al. Microsoft reacted like they often did: they created their own 'open' format, deceptively named 'Office Open XML' to make it easy to confuse with the OpenOffice XML-based format and submitted it to standardisation bodies in the hope that one would bite. ISO - which already standardised the Open Document format - did, giving Microsoft a weapon to wield in any bidding war on which file format should be adopted as a standard.
I called OOXML a boondoggle because that what it was. Microsoft's documentation of the format consisted of a several thousand pages long semi-binary dump of the files produced by Microsoft Office - which are not much more than memory dumps containing binary blobs, poured into an XML container with vague descriptions of what the data types actually were - often no more than 'as produced by Microsoft Office version x.y'. A large portion of the original OOXML files consisted of blobs of binary MS Office data, making it close to impossible to implement that version by non-Microsoft entities. When pressured Microsoft agreed that the current versions of Microsoft Office were not producing 'standard-compliant' OOXML but that this would change in newer versions. Well, the future came, the OOXML format was cleaned up and documented so as to be interoperable but Microsoft never adapted Microsoft Office to the standard, instead producing its own iteration of 'the standard'.
In short, the only reason why they every produced OOXML - the 'standard' - was to use it as a club to wield in negotiations. They never meant for it to be an actual data interchange standard, they never aimed at having third parties stand on a like footing with regard to which version of OOXML was meant to be canonical. It was, and still is, meant to be a box to tick on a compliance list for getting large contracts, not a file format standard to which their products would be tailored. Their vision is whatever is produced by the latest incarnation of Microsoft Office is 'the standard', just like before.
The real question to be answered here is why ISO went along with this charade when they already had a working file format standard in the Open Document format. They could just have said 'no' like other standards bodies did, Microsoft could have been made to implement the ODF format and the world would be a better, more interoperable place.
So indeed, Microsoft should not be allowed to use a file format of their own design when implementing their own software while selling it as if it abides by an ISO standard.