AM receivers have to apply a band pass filter to select the station. The width of that filter will directly affect the audio bandwidth. The filter probably can'

AM receivers have to apply a band pass filter to select the station. The width of that filter will directly affect the audio bandwidth. The filter probably can't be anywhere near +/- 20 kHz based on the idea that nobody nowhere would put two stations close together in the same area. Mass produced AM radios have to work everywhere.