AM means Amplitude Modulation.
FM means Frequency Modulation.
Lightning, electric motors, lawn mower ignition, car ignition, etc. all produce static that adds extra peaks of the Amplitude Modulation.
Lightning, electric motors, lawn mower ignition, car ignition, etc. they produce static peaks too but it has no effect on the frequency.
In the final stage of the circuit sound and video are seperated from the carrier wave.
In AM after the carrier wave is removed the static peaks are still there because they have become part of the video and sound.
In FM the static is there but the sound and video come from the difference if frequency change caused by modulation of the carrier wave so the static has no effect of frequency so it has no effect of the output.