There is painfully little on regular FM radio anymore in the US. It is becoming the new AM radio. HD FM is difficult to support with a tiny antenna, I would presume by the lack of support already.
Besides, most US carriers blocked the radio API in their phones, so many in the US did not even know it existed. From what I understand, FM radios are still prevalent in phones sold elsewhere in the world, like South America and Asia, where FM is still the dominant radio signal.
The rise of HD Radio (which I assume phones didn't support), streaming audio (Pandora, Spotify), and even radio apps like iHeartRadio made it obsolete?
There is usually at least WiFi everywhere you would reasonably expect to get an FM radio signal. When not on WiFi, even with capped data you can stream some form of audio. Barring that, you can store quite a bit of music on a smartphone, and get news in text form. Not much need for a radio on your phone.
Sony still includes a FM radio. The Xperia Z5 has one. As said elsewhere in this thread, though, their phones are more prevalent elsewhere in the world, not in the U.S.
Although I have a Sony Xperia Z3 and Z3 Compact, neither was bought through my carrier. While the Z3 does require headphones for the radio's antenna, you can play the radio through the speaker.
Probably because they'd need another antenna to support that, and in the age of thin devices, I don't think anyone wants to squeeze an antenna on for a feature very few people would use.
Besides, most US carriers blocked the radio API in their phones, so many in the US did not even know it existed. From what I understand, FM radios are still prevalent in phones sold elsewhere in the world, like South America and Asia, where FM is still the dominant radio signal.