The thing is, it's a misdirection. The 'so-called' secret is in how its cooked. Col. Sanders originally used a modified pressure cooker to deep fry the chicken pieces in. The main 'herbs and spices' are just salt and pepper and corn starch flour.
Plus if you get the public to believe that there is some kind of secret that only a handful of people know, it kind of adds some sort of mystery to the product. It's a sales gimmick.
Plus I think the real secret ingredient is crack-cocaine.