.doc is more stable, but it's also much more onerous to be fully compatible with. .docx was at least designed as a reasonably modern, rational document format with a specification. .doc by contrast evolved from generations of MS Word that for efficiency saved files by doing raw dumps of internal data structures as binary blobs, which got more and more complicated as things accreted over the years and versions. And there was no public documentation of the very complex binary format until 2008. Joel Spolsky has some on that here: https://www.joelonsoftware.com/2008/02/19/why-are-the-micros...

The JPEG file formats are nearly as bad, especially once you get into camera vendor extensions. It was common practice back then. Just take a bunch of C structs and dump them from memory to disk.