Having a thermometer that can measure ambient air temperature (that being, the air around you, not the air in/on the phone) would require having some sort of probe jutting out of the phone, which is not something people want on their phones. Putting one just on the surface of the case without jutting out a lot means that what it tells you the temperature is will be heavily impacted by the internal temperature of the phone, how you're holding it, etc.

Phones DO have thermometers internally to monitor how hot for example, the CPU is getting.

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.

Some of them do. My Cat s61 has an ambient temperature sensor with VOC sensor and humidity. It also has a thermal camera, and laser distance measuring tool. The ambient temp is usually a few degrees high when I'm holding the phone, but it's decently accurate when it's sitting on a table for a few minutes. My thermostat shows 72°f in my house right now, but the phone says 74°.

However, all these took up space that could have been used for other things, like a larger battery, better antenna, or a thinner profile. Basically, it boils down to "who needs this" and it's easier just to get local temperature information from the internet, or a separate tool if it's really important. As a result of this and the rubberized/reinforced body, my phone is large and bulky. That being said, for my specific use case I'm willing to make these tradeoffs.

For what purpose? Measuring the air around you at home? Well, almost every house has a thermostat, many smart that you can check on your phone or use a voice assistant. The check the temp outside? Do you want to go outside and let the phone acclimate to get a proper reading when again, you can check the weather app and it would be more informative and accurate? Okay, say you’re in a office building and don’t have access to the thermostat and you’re hot. I guess you’ll just be warm for a few minutes as they aren’t going to adjust the thermostat for you, not to mention it would likely take a few minutes to kick in and cool down.

Not to mention, due to the heat of the phone and it being in a pocket against your warm body, it wouldn’t be very accurate unless you put the phone in an idle state for a few minutes and leave it on a counter.

My Galaxy S22+ has one. It usually runs a little over the ambient temperature because you're holding the phone and the phone generates heat.

There's also humidity and pressure sensors. You just need an app that can read them.

A thermometer to measure what? Air temperature? Your mouth? Your steak?

Others have already covered this, but the problem is that the phone itself is a source of heat. It wouldn't be a very good thermometer for external usage.

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