Embrace, extend, extinguish.

Isn’t it open? It works with Dropbox and macOS Numbers opens .docx and saves as .docx.

Yes, it technically is a standard, but because it's an ISO standard you have to pay for I wouldn't call it open. https://www.iso.org/standard/71691.html but people have different definitions of what an open standard is and I'm not trying to critique them.

So it's an "open standard", not in the sense that anybody can contribute to the development, but in the sense that the details of the standard are open and you can learn about them.

The format itself is an XML version of the existing Office document formats, and they grew organically over decades with random bugs, features, and bug compatibilities with other programs. e.g. There will be a random flag on an object that makes no sense but is necessary for interoperating with some Lotus 1-2-3 files that a company had, who then worked with Microsoft to support back it in the 90s. Things you can't change, nobody really cares about, but get written down because the software already implements it (and will emit sometimes)