Some good ones are! But some add a whole bunch of extra ingredients.
But even with the mostly natural ready mixes, there's a problem in that salt and baking soda last pretty much forever, flour last for a long time, but baking powder goes bad after a few months. So all mixed up together, is only as good as the weakest link: the baking powder part.
If you mix your own, as long as you're using fresh baking powder every few months, you get higher quality pancakes.