The Samsung Galaxy S4 has both a thermometer and a hygrometer (as well as a barometer). Samsung removed both of those sensors for the S5 in order to waterproof the phone - they were able to keep the barometer while still waterproofing, which is neat. Someone else might have to answer how that part of it works (something about watertight and airtight having different physical requirements..?)

In fact, every phone does have a thermometer, but that is not usually accessible by developers and it does not measure the actual air temperature - it's used by the phone to make sure it doesn't get too hot from the battery or CPU. We might not see many app-level thermometers get added to phones because the data is extremely difficult to use. For example, most of the time it will measure the temperature in your pocket or in the room where the phone is - and even then, the reading will be radically affected by what the phone is doing (browsing the web is going to cause a different temperature reading compared to watching a video, compared to stand-by mode).

I do expect that we'll see more hygrometers and thermometers and other sensors make their way into phones as time goes on, but I think the primary reason for the slow take-off is because the data is really really hard to use. That's the thing with the barometer - it's very resilient to noise and even when the data has problems, they are fixable.