Earlier quoted context omitted.
The answer is simple, it's just kind of says an ugly thing about how computing has shifted; it's essentially the same reason e.g. the iPad didn't have a file manager. Interfaces got really good and slick at the same time it became profitable/useful to alienate people from their own data for profit.
Files have some drawbacks... they're difficult for multiple people to work on together (slides). They're often not easy to organize, and they're often not available where you want them.
If you use something like Syncthing[0] (great, can recommend) you'll get them available exactly where you want them: on all your devices.
The only real problem with it is conflicts, where one file is edited simultaneously on two devices. I think Dropbox has solved that (?), but I'd rather keep my data on my own computers.