Well, I'm not a tire engineer, but a slick tire still has more contact patch compared to a DOT. Yes, in this day and age, the DOT's are pretty good, and they are directly related to the slicks. Back in 1990, there were DOT tires that were the same construction and compound as the slick tires, but they were put in different molds.
The tire companies want to sell tires that street bike riders use. Makes their tires seem better to the general public.
Even then, there are so few "superbikes" at the club level, why do we still have "superbike"? Sure more races, but why not longer races for a cash purse. Would make the whole program a little more interesting, right?