Why is the Neutral and the Ground Bus-bars bonded in a Service Cabinet and they are not in a sub-pannel?

Question

Why is the Neutral and the Ground Bus-bars bonded in a Service Cabinet and they are not in a sub-pannel?

Answer 1

because the sub panels are not connected directly to the external ground rod.

Answer 2

the nec said the neutral shall be grounded only at one point, at the point closer to the source.

the grounding wires is not supposed to carry current, if you ground the neutral at sub-panel, when the neutral is broken, the current will continously flow on the ground wire without tripping the breaker, this is unsafe.