I think the "why" is probably that if you want wired a 3.5mm jack and an adapter works better for most people than dedicated USB-C headphones. That way the same headphones can be used in other devices with a headphone jack (other phones, portable music players, my laptop), and you can choose the headphone and adapter that suits you best, rather than being restricted to the available USB-C headphones. If you already have wired headphones an adapter is cheaper than a new set, especially if you have good headphones (though if you are fussy about the sound you can buy a higher quality adapter to match), and most adapters can just be left permanently on the headphones, in which case there's little convenience lost over a dedicated USB-C headphone.

I've got 2 pairs of headphones I use (a smaller set for commuting and a larger one for longer use), and wasn't going to replace them just because of a dick move by phone makers (I've never believed that it was a coincidence that Apple bought a popular headphone brand shortly before they removed the headphone jack from their phones). I've got regular £10 adapters I keep with each, plus a fancier one I keep with the better pair. That works for me, though frankly just having a headphone jack would work better.

How well that works as an FM antenna I don't know, as my phone doesn't have that option.