I don't understand why nobody else is able to do it...
Because they don't care, need to, or want to? Google and Microsoft are competing with Dropbox on economics of scale and perhaps as a loss-leader. A lot of people are willing to forgo peer to peer LAN sync or block-level syncing when the cost is much lower. Additionally, both Google Drive and OneDrive have an integrated office suite, which adds value for a lot of people.
Moreover, if you can get away with a weak client, it's an advantage. Someone who has all their data synced to disk can easily switch. Someone who has their data primarily in someone's cloud cannot as easily.
Because they don't care, need to, or want to? Google and Microsoft are competing with Dropbox on economics of scale and perhaps as a loss-leader. A lot of people are willing to forgo peer to peer LAN sync or block-level syncing when the cost is much lower. Additionally, both Google Drive and OneDrive have an integrated office suite, which adds value for a lot of people.
Moreover, if you can get away with a weak client, it's an advantage. Someone who has all their data synced to disk can easily switch. Someone who has their data primarily in someone's cloud cannot as easily.