FM did not exist at the time aeronautical radio began. In fact only AM was available until the early 40,s and not used much at all until after WWII. Even SSB wasn't practical back then due to frequency instability. With FM, the strongest signal wins, which is the FM capture. AM signals mix with two or more AM transmissions, although somewhat garbled you can still hear two transmissions and often figure out some of what is said. AM is considered somewhat safer for aircraft to use than FM, but there is a desire to go digital on aeronautical to get more channels which negates the benefit of hearing mixed AM. The problem is replacing all those radios being used poses a huge economical issue and therefore safety issue. I doubt we will see digital for a very long time. When aeronautical radio went from 100 KHz channel spacing to 50 KHz, 25 KHz and then 8.33 KHz, it allowed some attrition time for using older radios. Digital will be a much bigger transitional challenge.