No one wants to pay extra for a feature in their personal car that will prevent them from exceeding the speed limit. (Some trucking companies do versions of this, but for personal cars, you might as well offer "seating with embedded broken glass" as an option.)
It makes sense when you put it that way. Still beats me how there isn't a regulation on this. Manually over-speeding vs being able to let my car drive itself at 100mph does not make sense.