[Radio Wrangler]

Several reasons:

Aviation went onto VHF rather early simply because of size of antennae and available frequency range. This was before FM got going for PMR and VHF marine users. There is so much infrastructure in aviation VHF AM radio and it would be such a nightmare to change it all simultaneously that they never bothered. Most aircraft wouldn't have space for duplicating equipment over a change-over period.

Along the way, the heterodyne tone was cited as a reason because it's an indicator that there were two (or more) transmissions overlapping. This isn't somethiing to rely on. The beat note could easily be below the 300Hz LF cutoff most radios have, or above the 2.8kHz cutoff the later standards require them to have.

The reason for the 2.8kHz lowpass rolloff is that someone invented a scheme for multiple transmitters on the ground, say along a long valley, all on the same channel (I didn't say frequency) The transmitters are actually offset several kHz from each other and transmit at the same time. The plane RX gives a horrendous heterodyne whistle as the plane flies in the regions where there are two strong signals, but the frequency is above the required audio frequency. The old '25kHz spacing' channels can fit 5 different TXs in, the new 8.33kHZ spacing channels can still fit three in.

The logistics of changeover would be inconvenient. There is already a change underway of replacing the radio in every aircraft with an 8.33kHz model, but that can be done gradually with dual-capability radios. The beat note means someone's doubling is useful but not reliable. The multiple ground station trick is the real killer.

David (designing aircraft radio gear is the day-job!)