Qualcomm has a massive "value add" because they own the modem. As well as a doom stack of patents on all things cellular.

You need a modem if you want to make a smartphone. And Qualcomm makes sure to, first, make some parts of the modem a part of their SoC, and second, never give a better deal on a standalone modem than on a modem and SoC combo.

Sure, AMD could make their own modem, but it took Apple ages to develop a modem in-house. And AMD could partner with someone like Mediatek and use their hardware - but, again, that would require Mediatek to prop up their competition in SoC space, so, don't expect good deals.

Not every scenario for such chips is a smartphone, but as you said, AMD could as well develop their own modem.

I would prefer them to start with WiFi though, since Intel made their latest chips impossible to use with AMD CPUs.