Because he has no access to a physical Grass type move, and the only flying move he has access to that's physical is Ariel Ace.
Which you have to look at this calculation.
Ariel Ace - 60 Power, when super effective power doubles to 120.
Earthquake - 100 power, due to STAB ( Same type attack bonus ) becomes 150 power.
Meaning Earthquake on Excadrill is actually better agaisn't a fighting type than Ariel Ace.
That's why they don't include Flying / Grass moves.