Going all digital in TV was to pack more signals in the RF remaining below RF51 after the 700MHz band was removed from TV broadcast...HD came along for the ride
Remember most TV stations had second signals on the air to carry their DTV in the beginning and most people watched TV via cable or sat..cannot do that with AM...
ALSO TV is a fixed service (in the majority of cases...you dont see a single person driving along watching TV in the car..in some states, its ILLEGAL to have a TV on in the front seat where the driver can see it..Texas is one). Radio is highly mobile/portable..apples and oranges
TV converters could be easily made by demoding DTV and sending it to S-Video/AV IOs/HDMI OR using a channel 3/4 modulator and a short cable (this was capable due to the fact there has NEVER been an analog 3 AND 4 in the same market....but 4 and 5 were separated enough to allow such in many markets). You cannot find an external antenna input on ANY portable AM radio (except for some rare ones)..and NO external audio inputs..thus making an external digital decoder worthless (in fact, there WAS an external AM Stereo decoder made but it had a FM modulator...because you cannot easily unplug a car AM antenna and insert such inline...think about it...while you reach inside YOUR dash to do so....average joe Q Public will not do that)...
I rest my case.