Writing a simple file sync utility isn't difficult, hell you could just wrap rsync and you'd be half way there. Making that open source is easy and low risk, because you haven't really invented anything.
On the other hand, if you can throw 100 developers at the problem, you'll come up with something _far_ beyond the simple case. Sure it might only be 50% faster? But that 50% makes your product "magic" when compared with other tools. This is where you get things like Project Infinite (https://blogs.dropbox.com/tech/2016/05/going-deeper-with-pro...). I can imagine there are some very complex optimisations for the multitude of configurations that Dropbox runs on, that I can understand a company wanting to keep "secret" if it's so core to their business.
I think you'd be surprised at the depth to which these companies are solving problems.
