Because phones get hot.

They do have thermometers. Several of them in fact because a lot of integrated chips come with them. But they tell you the temperature of the chip in order to prevent it from overheating.

In general, you’re not going to find an external temperature sensor in any electronic device that isn’t designed for it because you need a large empty space around it — and phones are jam packed with battery capacity and antennas wherever they can fit them.

Any temperature sensor is going to be more likely to tell you the temperature of your screen, battery or hand than of the room it’s in.