I has been trying to reverse-engineer the Microsoft Office file formats and I've found they are actually Open XML and the "Open XML" file format is actually a ZIP format.
When I open the Office files with a binary (hex) editor, it has the same file signature of a ZIP file (the signature is
By the way, the Office files can be opened with WinRAR like any ZIP file.
My question is: Why Microsoft decided to use ZIP containers and XML instead of a proprietary file format with custom signature and headers? Doesn't it make easier to reverse-engineer or slower to read and write?