The gain works both ways of course. However, typical airborne units will probably be 10 watts, whereas ground should be at least 20 and potentially 50 or more watts - so they shout louder and can be heard farther than they can hear.
Equally, a good ground antenna (assumed to be a given) and a good airborne antenna (much less obviously true in typical spamcans) will work much better than the same ground station with a typical 'club' airborne installation.