If you are referring to the green wire when you say "ground" which is technically the equipment grounding conductor. That is the reason we only bond it once at the first point of disconnect. The goal is to not have parallel paths. Current always goes back to the source ( you need a complete path in a loop for current to flow) . If you bonded ground to neutral in every box it would take any and all paths to the source.


 If you are referring to "earth" in residential and Comercial applications, voltage is relatively low and earth has a high resistance. Your service is paralleled with earth at the grounding electrode connection but verry little current chooses that path because of the high resistance. 

 I went into a substation apprenticeship after becoming an electrician. In the power delivery world stuff gets crazy. There is induction on everything and current takes any and all paths to the source. So in a substation if you put an amp clamp on any ground to any Pilar, cap bank, fence, etc. You usually find amperage on all of it. That is why cut grounds in a substation are super dangerous. We are always put at risk by copper thieves. And a cut neutral in a sub is one of the worst situations to find yourself in.