[davekeck]

I always assumed it was because FM station bandwidths (200kHz) are much wider than AM (10kHz). AM's 10 kHz chops off a lot of human-hearable frequencies.

[ndndjdjdn]

AM doesn't use the frequency for modulation though so it shouldn't matter.

[kragen]

When you amplitude-modulate a carrier wave with an audio signal, you spread it out into a bunch of sum and difference frequencies, as you can see if you use the trigonometric angle-sum formula to factor cos(85000·2πt) · (2 + cos(440·2πt)), a 440-hertz flute being transmitted on 85-kilohertz AM. These so-called "sidebands" mean that the bandwidth of AM does matter, and consequently, using a too-narrow bandpass filter on your AM radio station will result in low-pass filtering your demodulated audio signal.

[t-3]

AM does use the frequency, it just doesn't need as much and uses it differently than FM. If it was all at a single frequency, there just be a single tone getting louder and softer.

[KK7NIL]

Other comments gave a nice explanation of why AM does need a bandwidth, but here's the information theory explanation: https://en.m.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theo...

TL;DR: the information one can reliably send through a noisy channel (C) is proportional to the bandwidth of that channel.