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.