> They don't do anything that their competitors (OneDrive, Box, Google Drive, etc.) don't do better.
I'm not sure on what you base this opinion. We do sync much, much better, and that's no coincidence--we work very hard on it (I'm new on this team, so I deserve essentially zero personal credit). It is our business. We're not an advertising company--doing sync well translates directly to dollars for us.
On a technical level, it is extremely nontrivial to get right multiplatform, in the face of races, vague filesystem semantics, application-specific behavior, offline, etc. Our competitors aren't even trying things like infinite (https://blogs.dropbox.com/business/2016/04/announcing-projec...).
We're not sitting still, though. We're actually building a next generation sync engine right now (btw, more Rust @ Dropbox). The plan is that this will put even more distance between us and others.