Oh but they can, if you have google drive in your desktop you'll see that they are a .gsheets document for example.
Or, even better: open a .gsheets or any .gEXT file in your favorite text editor. You'll see that they are comprised of web markup language, much like an XML doc. I don't have one in front of me atm, but I recall thinking, "it's basically XML that points one's browser to the appropriate Google app and a unique ID for the document."
[^1]: https://support.google.com/docs/thread/10078336?hl=en [^2]: https://support.google.com/docs/thread/10078336?hl=en&msgid=...