Unfortunately your explanation doesn't hold water if you consider other instruments.
C major / A minor are even easier on piano. (C major and A minor are relative keys – they contains the same notes, just with a different note emphasized as the root, or tonic.) They're the two keys that are made up of just playing the white keys on the piano. In contrast, G major / E minor is probably the easiest key to play in on guitar (containing G, C, D, Em and Am), and it comes in second place.
C major / A minor are even easier on piano. (C major and A minor are relative keys – they contains the same notes, just with a different note emphasized as the root, or tonic.) They're the two keys that are made up of just playing the white keys on the piano. In contrast, G major / E minor is probably the easiest key to play in on guitar (containing G, C, D, Em and Am), and it comes in second place.