Why isn't there a ground in a junction box?

Only the main panel is grounded directly because if you ground each junction box of subpanel ground loop current paths are possible that can cause shock hazards. The ground rod is connected to main panel and the neutral is bonded to that ground as is the ground wire. The neutral and ground wire are then run to all circuits, junction boxes and subpanels throughout the house.

If you tie neutral and ground together at any point in the house, other than the main panel ground loops can exist and cause shocks.