Most smartphones do include most of the FM radio circuitry in the chip used also for Bluetooth connectivity for example. It's just a choice of the manufacturer to do not implement it completely, sometimes in hardware, sometimes in software. Several years ago there was SpiritFM application for Android that allowed to use FM radio on smartphones that sometimes never supported FM radio... oficially. So the only missing piece is sometimes only the software. As a result I think it wouldn't cost somewhat noticeably more to have that feature.
Also, I believe that is exactly the reason why FM tuners are being phased out.
Streaming services will nowadays give you uninterrupted playback pretty much anywhere, even in places like large buildings or subway tunnels, where radio coverage is nonexistent. Basically it all boils down to the fact that cell network coverage is much better today.
I don't know about "most smartphones," but the FM bits were commonly a built-in with Qualcomm and some other chipsets for a coon's age.
Even the OG Motorola Droid, of 2009, had the parts inside for an FM radio. They just weren't connected to an antenna and the default software stack didn't include any access to those parts at all.
Flagship Qualcomm-equipped Samsung phones up to the S20 or so tended to be ~similar: The FM tuner guts were usually in there, but they often weren't available to use, and even then they sometimes lacked an antenna.
This is not intended to be an exhaustive list. :)
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In my time, I've enabled FM radio on a few different phones that weren't advertised to have that function, sometimes with root-only hacks from grimy corners of the internet, and sometimes by just installing some software that just happened to have the right hooks.
Twenty years ago this would have been wrong with a kernel of truth. Today, it's just wrong. There were chips that integrated all this but they weren't that popular in the West and ultimately nobody cared.
And it's not a tiny kernel of truth, even iPhone(2007) had Wi-Fi/BT combo chips with FM radio. Apple was partly responsible for death of FM radio, by not implementing the software feature. That's also literally 20 years ago.
