This has been suggested. The reason for not having a Category 6 or 7 is the scale measures damage to man made structures. No building can take 160+ mph winds for more than a few seconds without serious damage. Once a building is completely wrecked, how wrecked it is no longer matters to how much damage it can take.
Maybe they should alter the categories to use natural phenomena like wind speed or something instead of rate of damage to man built structures.