And that’s exactly why they’re billionaires and you probably aren’t. It isn’t about functionality, it’s about ease of use. It’s the same argument used when MS-DOS was being replaced by the GUI.
Explain FTP to your Aunt Edna. Get FTP working where everything on all devices is in sync automatically. Comment on a file via FTP. Send a link to a file they can consume on a mobile device — including playing audio or video without downloading the file. Create access lists for folders.
Sure you could do it using FTP. Now do it cheaply at scale and make it work everywhere and then convince your non tech friends to use it.
Why bother with GPS when we could all use a map and compass? Dropbox made file syncing accessible to everyone cheaply and securely.