Exchanging and syncing files is very tricky service to get right. Because no one has managed to do it painlessly so far. It is much more than just users and hardware.

While dropbox are of the better services out there it is still open for innovation - there are still pain points to be eliminated. So squashing them may require another startup and not big behemoth.

As with the case of Origin and Windows 8 learned recently - creating something people like is just hard.