I was trying to explain this the other day to a client…
Mr. Pope can correct me if I’m wrong (I consider him the expert).
You can connect neutral and ground at the Main service panel because that is the “junction point” where electrical service enters the house. Anything downstream of the main service panel has junctions which may become loose or inadequately connected allowing current to flow back into the neutral, rather than the ground due to inadequate path of least resistance to ground.
There is only one point of connection in the Main service panel for each component. Once you extend the electrical conductors to subsequent service panel’s, you increase the potential of increased resistance which may cause electrical failure to back feed on a path of least resistance to a non-desirable location.
The main service panel has two legs of power, one neutral to the transformer, one grounded electrical conductor (to a grounding rod, hopefully below the meter box), one grounded electrical conductor back to the main transformer feed, and possible other connections depending on design.
Any interpretation that electrical current does not flow on neutral lines is a fallacy! It will in fact get you killed! Neutral wires are not on the same level as ground wires. They do carry electrical current where as ground wires do not. If you have an electrical failure which causes current to flow back onto the grounding system, you want the least path of resistance to be on the grounding conductors rather than the neutral.
Neutral wiring carries electrical current, therefore reducing its capacity to provide a line of least resistance back to earth where the electrical current does not cause physical injury. Grounding conductors should never be considered "the same as " neutral conductors.