The answer to 'why is my water heater not cooling' is that the thermostat or temperature control on your water heater is likely not functioning properly. This can cause the water to stay too hot, preventing it from cooling down as it should.
Some common symptoms of this issue include scalding hot water coming out of the taps, the water heater running constantly, or the tank never reaching the desired temperature setting. To fix the problem, you may need to adjust or replace the thermostat, or there could be a more serious issue with the heating element or other internal components.