The real reason is SC is the cash cow of the hobby right now. Most of the people that SC brought into the hobby are blissfully unaware. SC guys want it easy. They see that Johnny Fastguy is running an AKA tire, but they don't have the experience to ask what insert Johnny Fastguy is running, so they just figure they must be the ones that come with the tires.
If you are going to do something smart (re-using inserts, running the right insert for the track conditions) concerning a SC truck, you can't get caught up by a company catering to the huddled masses of the SC truck market (the guys that have no idea, and blissfully plunk down an extra $7 because they don't know any better).