Because coolant/antifreeze was developed to work in both extremes of temperatures so no need to develop a hot weather only type of coolant. It just happens to have the benefit of both.
If you are having cooling issues, there may be an issue with the cooling system. We have members here from Saudi Arabia that experience temps in the 40ºC+ range. To help the system, there are additives, like Water Wetter, that reduce surface tension of the water/coolant mix to aid in heat transfer, which can be useful for extreme situations like towing in extreme temperatures. I use this, oddly enough, in my snowmobile to help keep temperatures down.