Microsoft was successfully sued by the EU for creating vendor lockin with their proprietary file formats. It is for this reason that the "X" formats (docx, pptx, etc.) are "open" and thoroughly "documented", e.g.:

https://learn.microsoft.com/en-us/openspecs/office_standards...

However, the formats are incredibly complicated, because they evolved from earlier formats that represented nearly the entire in-memory state of the editing software. To a first order of approximation, the .doc format _was_ Microsoft Word.

Source: I worked at Microsoft during this time period and helped document the XLSX format.