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Sandstorm is a rock move and and buffs rock types. However all other sand-based attacks are ground type and more Pokemon with sand related abilities are ground rather than rock.
Would you object to Rock and Ground being condensed into a singular Earth type, or do you think they're different enough to warrant 2 types? Or do you think that even if thematically it would make sense, the balancing would be a nightmare?
What if it was like:
Weak - Water, Ice, Grass
Resists - Fire, Normal, Poison
Immune - Electric
Nulled by - Flying