Why don't voice radio communications use FM?

Stefan wrote:
Mxsmanic schrieb:

I don't see why it would be so objectionable. It isn't even necessary
that the AM be phased out. The FM would simply be available to those
who wish to use it, for the added clarity it provides.

Yeah. One pilot talks on AM and the other listens to FM. Great idea.
Adds a lot of clarity to the communication.

Huh? How is that problem different from something transmitting on 121.5 MHz
and someone else listening on 406 MHz?

Why is having multiple channels all using AM not a problem but if one added
more channels using a different modulation now suddenly presenting a
communication problem?