I can build "my own Google Docs" with a series of many different document store databases. Why would I have a proprietary API back-end I cannot host myself as

I can build "my own Google Docs" with a series of many different document store databases. Why would I have a proprietary API back-end I cannot host myself as an alternative to Google? Let me host my own live text editor, text, editor, and all, xthxbai. The irony is pretty funny though.