Listening to DAB on a cellphone requires a monthly data plan.
If you’re listening to something on your phone through a data plan, it’s not DAB, but an unrelated streaming service.
Even though older iPhone baseband chips had FM receivers, you’d still need break it out and deal with the antenna setup, with the approach taken by others requiring wired headset and shenanigans in the headphone port. That’s more BOM, and user resistance to wireless headphones or newer models whose baseband chips cut the feature.
It used to be a somewhat common feature in phones (including smartphones from other brands), but it usually had quite “eh” reception. And with transitions to DAB, the feature becomes unusable without DAB decoders which I’m sure someone is claiming royalties for.
Cellphones used to include AM/FM radios.
Personally I think it might be useful to have them for public service uses: perhaps cell network are more robust than a few years ago, but there are still instances when they could be (are) overwhelmed, and a broadcast-only system could be useful in major emergency events.
A cell network doing a one-way broadcast text message to all clients is a solved problem. And honestly, I greatly prefer this way to push out an emergency message than assume I'm tuned to a radio station or broadcast TV, two things I'm rarely doing. I'm way more likely to be listening or watching local or streaming media instead of watching broadcast TV. Or even just not be doing any of that at all. Lots of people I know wouldn't even be able to tune into a broadcast TV or FM radio station outside of their cars.
Smartphones a lot* of antennas, but most target 1/10th the wavelength of FM radio.