Actually a subwoofer in a phone wouldn't be impossible. With the ability to create bass channel tubes inside the case, you could probably get some real air moving with a decent sized transducer. Still, you may add SOME lower register sound but you'll never get the kind of bass response out of them that you do from even the smallest portable Bluetooth speakers. It's just a matter of physics and air flow/air volume. You need to move large amounts of air to create bass.
One way they could overcome this limitation is to go to a radiating panel for the back cover and have that be the bass transducer. I have a pair of flat panel speakers that are battery operated, and use corrugated plastic panels as the transducer.
The bass response is pretty surprising considering there is no actual "cabinet" or air channels/tubes to enhance and amplify the air movement. The way it moves enough air to make real bass response is simply by the square inches of surface moving that air.
By making the rear panel of the phone (or at least a portion), "floating", perhaps in a rubber grommet around the perimeter, and then putting the transducer on the inside of the phone attached to the inside (back), of the real panel, phone manufacturers could use it as the transducer. This would yield some additional air movement and increase bass response.