They simply don't know better. You'll only see gains from a 26" tire once you trap over 150MPH. This I am quoting from super fast guys in the DSM world who have already been there and done it.
Personally I don't plan on making my car trap over 150MPH. Even if I did, I wouldn't buy a 26" tire untill I actually got to that point.

You simply need as much tire as your car requires. The bigger the better doesn't apply here