Why if I'm a raider attacking a farm account, it doesn't bring resources but money?

When you attack another baser you will raid resources (and money if you use the Raider Class APC) from the enemy depending on your APC's capacity but only if they have available resources to take, this does not include any stored resources in their items.

If they are a Farmer then they will have a protection level on all resources that will protect a certain level. Their alliance (if they are in one) may also add additional protection.

I would guess that the account you are attacking doesn't have enough resources for you to take.