>There are also many FOSS applications, including LibreOffice, that are perfectly fine as replacements for Microsoft Office.

From my own experience dealing with MS Office-based government organizations, this is absolutely false. Sure, LibreOffice can read and write DOCX, XLSX, and PPTX files that are nominally interoperable with MS Office. But more often than not, MS Office mangles the formatting of LibreOffice-generated files. For example, text sizes, fonts, and element alignments in PPTX files get thrown off noticeably.

As another reply mentions, there's this nominal open-source standard. But the de facto standard is the MS Office implementation. I have to use MS Office instead of LibreOffice to eliminate the risk of document mangling when I send files over to the government people in charnge of my funding.