As you pointed out, your phone doesnt have a thermometer. Or, it does. But it is for checking for overheating. But if your phone tried to measure ambient air temp, it would actually measure temperature inside its case, temperature in your pocket, temperature of whatever it is sitting on, etc.
And even if it did have this function, I typically don't care about the current temp. I care about what temp it is going to be, or if it will rain, etc. I want a weather forecast, not information about the weather around me