If it were as simple as you imply it is then surely we'd see this change made trivially to SMB, NFS, WebDAV, or SSHFS.
I agree with you that in hindsight this is an obvious feature, but they actually decided to implement it, and I bet it wasn't entirely trivial.
The biggest problem, as usual, is invalidating the local cache. For Dropbox, who own and implement the authority on the shared drive state, it is a _very_ (relatively speaking) simple problem.
Other protocols like SSHFS have to deal with filesystems of all kinds. Many of those do not support anything like inotify, and polling over huge directories would be horrible experience or performance wise (long delay or slowing down the whole host machine).
I first had a good look at caching distributed filesystems (and tried to build one) in my undergraduate thesis in 1999.
In hindsight? It's about time! I've been waiting for this for years...