Found this on a type-r forum,amongst other stuff about oil, one part stood out which ive underlined:-
"Although the basestock of an oil will be a major determining factor in the lubrication quality of an oil, chemical additives play a major part in making sure that it does all that it is supposed to do. The chemical additive package of an oil is just as important to insuring the quality of a lubricant as is the particular basestock used.The chemical additive package of an oil is designed to perform a number of tasks and each task is performed by a particular type of chemical. The quality of the chemicals used and the manner in which they are blended plays a large part in determining how well the additive package does its job.
As the quality of the additive chemicals increases, so does the price. In addition, proper blending takes a great deal of research. This requires much time and, again, money.
Therefore, manufacturers will, of course, charge more for motor oils which contain a high quality additive package than those with lower quality additive packages. They simply can't afford not to.
Each chemical within an oils additive package plays a different role in boosting the beneficial properties of it's host lubricant (basestock). "
So maybe it is a cost thing,perhaps companies would add it if people were prepared to pay the price which they aren't or maybe they can't add it because they don't own the patent for that additive.
Or maybe not who knows,im prepared to give it a go anyway.