Remote panel grounds and neutrals in main panel?

Out of curiosity, I saw a 120v subpanel, next to the main panel, where the subpanel circuit neutrals and grounds remained in the main panel (bonded). The subpanel is grounded to the main panel by a 6" metal conduit. A 60 amp double pole breaker in the main panel feeds the sub and the sub has a few 20 amp breakers in it for separate new circuits in the building. I know the NEC requires separate neutral and ground bars in a subpanel. WHY does the NEC not allow neutrals and grounds for a subpanel to stay in the main panel when it is on the same building?

 

Are you asking why you cannot run 5 ground wires and 5 neutral wires for 5 circuits through your 6 inch long 1 inch emt, rather than connect to terminal bars in the subpanel?

 

NEC needs to cover all installations to prevent people from causing other issues.
1 - Complexity. Whenever maintenance is done you want all conductors in one place. Too big a chance of someone thinking wire not used as its hot is missing.
2 - Ratings. If needed to move to a sub panel that is normally because out of space in the main. If out of space for circuits probably also out of space for grounds and neutrals - at least for what the box is rated.