look at it from the other side.

If you and the people around you do everything right, you won't hear flarm.

If you or somebody around you makes a mistake you will hear the alarm.

The trick is to keep the instrument silent. This means there is zero extra cockpit load if you keep a proper look-out.

People against flarm are worried that there view of themselves as a "superpilot" that never needs instruments is going to have a rude awakening, because the sound of the buzzer means that somebody failed.