A family of Microsoft word processing software products for creating web, email, and print documents.
Well Zian, I think the same than you, but at the moment Microsoft just has this option for OneDrive. I not sure about they reasons, maybe commercial...
Microsoft says:
"AutoSave is enabled when working on a file that is saved to OneDrive, OneDrive for Business, or SharePoint Online. If the file is saved to another location (or if it hasn't been saved at all, yet), then AutoSave is disabled. This will happen if your file is on a SharePoint on premises site, a file server, some other cloud storage service, or saved to a local path like C:."
https://support.office.com/en-us/article/what-i...
I suggest you to send your feedback to UserVoice:
https://office365.uservoice.com/forums/264636-g...