 Because if you don't keep that open and accessible for the cooler air, even if the cooler air is 90 degrees, it's cooler than 130 in that attic. And that cooler air will allow the hot air in the attic to escape up through the ridge vent or gable vent or box vents on the roof because hot air rises. Hot air ain't going to rise until cooler air is pushing that hot air out.