[he can't even begin to react to how wrong he thinks you are, Asch, they are totally the greatest]
No one type is the greatest, actually. They all have their own individual strengths and weaknesses. Most trainers tend to raise a variety of types, but that doesn't give you as much of an opportunity to understand the nuances of one particular type.
Furthermore, it's not as though Ground-type Pokemon can only learn Ground-type techniques. They're just at their best when they use them.
[voice]
[he can't even begin to react to how wrong he thinks you are, Asch, they are totally the greatest]
No one type is the greatest, actually. They all have their own individual strengths and weaknesses. Most trainers tend to raise a variety of types, but that doesn't give you as much of an opportunity to understand the nuances of one particular type.
Furthermore, it's not as though Ground-type Pokemon can only learn Ground-type techniques. They're just at their best when they use them.