That's only weird to those who think open standards to be
  the only viable standard.

Open standards are the only viable standard if you want adaptable, future-oriented and collaborative software ecosystems as well as likewise markets. You simply cannot guarantee or even create these circumstances with standards that are set by a single corporation (or worse, a trust) - which is only logical because they were designed to do the exact opposite ('defective by design').

  MS Office is the de facto standard

I wasn't arguing that. My point is that this is bad and needs to be replaced.

  your data is not vendor-locked when using it

I think you somewhat misunderstand the term 'vendor-lock'. Sure, you can open Office files with other programs and convert them into open file formats, such as odt.

This is, however, mostly thanks to people reverse engineering Microsoft's original binary file formats, and MS was not really happy about this to begin with. If they could have prevented it, they would have done so (and they tried). Even the newer OOXML is not entirely documented and prevents free implementations due to patents (which, no matter what Microsoft may claim, is the exact opposite of an open standard).

Also, while this conversion might work fine for simple, small documents (or other files), the more complex and larger your filed become the more impossible it becomes to convert without a major hassle, which brings us back to your misunderstanding of 'vendor-lock'. The terms doesn't necessarily mean that it's impossible to switch to alternatives, but also applies when measures are taken to make it as hard as possible to switch without investing heavily in time and money.

As a side note, I am not attacking MS Office specifically. It's just the best example for showing all that is wrong with closed standards and proprietary file formats.