Although they all seem to do the same thing, each company has developed them separately, so each has spent differing amounts of time and resources into sync.
Also, AMD and Nvidia are the ones who develop, manufacture and sell the sync modules to display manufactures. so they get to decide the value of the chips. Nvidia is just more confident that they have the better sync, so they can sell it for more. Whether or not it is better, we keep paying for it because most people have Nvidia cards and there really isn't a good alternative.