My guess is because it's not physically possible given the way our ecosystem works even if it is theoretically possible.. At night temperatures drop and greater than 100% humidity isn't possible. This same air heating up during the day will by definition have less than 100% humidity unless there is a source of new moisture. Unless there is precipitation or a large body of water then there is no source of new moisture. Note that both those things usually have a moderating effect on the temperature. Hence, we're back to lower temperatures along with the higher humidity.
Another guess is that humidities above the blanked out ones have little additional discomfort effect much like winds over about 45 mph increase the wind chill factor very little.
