Ok, So Why Is Electric able to do damage on rock
And Not Ground.
Because Game Freak wanted to. :P
I think because electric is able to crack open a rock. Ground just absorbs the shock.
Just my theory.
Hope This helped :D
good question i belive the reason is that most rock type also have a different type (For example tyranitar is rock and dark) so instead of making all them so complicated they decided to just make it like that... and this is just a theroy so plz no negitive comments
Well, in life electric can break rocks into pieces.
(credit to dr.flame for that)
In life then electricity can build up in ground and shoot out proving the answer.