The Foundation sees this as a contradiction to the EU’s own interoperability goals. Although XLSX is standardized as OOXML according to ISO/IEC 29500, Microsoft’s implementations often deviate from the specifications. Furthermore, features often change undocumented, which complicates compatibility with open-source software such as LibreOffice.
csv is a pretty good data sharing format, but not very well suited for spreadsheets. Just because you can shove anything you want in there doesn’t mean you should.