Because the weather swings back and forth, hot and cold. When you put more energy into it, these swings get bigger but also trend towards hotter. This means that you get hotter summers and occasionally colder/wetter winters, but each summer will be the hottest summer of your life from now until you die. Worse: a lot of our infrastructure relies on weather patterns and ocean currents, and those are greatly influenced by the evaporation of water in hotter locations. With greater heat, you melt more glaciers, ice caps, and ice sheets. This additional water will break a lot of these patterns, and will eventually make some parts of the world (like Europe) that are very amenable to human living into places that can no longer support agriculture.