It is no surprise, when one-product companies do other products they don't last for long.
Only reason why I still use Dropbox is their client for Mac and and simple web interface and some other small features. I don't understand why nobody else is able to do it...
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.
Network file systems are hard. There are 30+ years of posts in various places of the Internet, hundreds of books, and dozens of implementations' source listings that you can read.
Dropbox have exceptional engineering talent, and a mature code base. Nobody else before or since, has ever come even close to what they have, quality- or feature-wise.
That they make it look easy doesn't mean that it is easy. It's like saying, all this guy ever does is hitting the puck in the net skating backwards, I don't understand why nobody else is able to do it?
Only reason why I still use Dropbox is their client for Mac and and simple web interface and some other small features. I don't understand why nobody else is able to do it...