Why don't Excadrill movesets include a Grass/Flying type move to counter weaknesses?

Question

If Excadrill is weak to water and Fighting types/type moves, why don't most movesets include a grass or flying type move to counter?

Answer 1

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.