Why are ground and neutral bonded at the main panel?
The reason they're allowed to be mixed in a main is a convoluted tale.
In principle they're supposed to be separated there too; however panels need a feature to keep 120/240V power from floating at some unnaturally high voltage to earth, e.g. 9600V from a transformer leak. That would cause problems for insulation in devices.  So they take one of the active conductors, name it &quot;neutral&quot;, and bond it to earth. The transformer then forces the other conductors to be the appropriate distance (120V) from earth.  This bond is called the Neutral-Ground Equipotential Bond.
It would be perfectly OK if neutral and ground were merely close; so for instance a 1-volt transformer would be a fine N-G bond, giving neutral a 1-volt bias from ground and the other legs 121V and 119V biases.  That is certainly better than the alternative, a 9600V, 9720V and 9840V bias from ground!  However a piece of copper is the cheapest available bond.
At that point, manufacturers said &quot;Wait, since they're all connected anyway, why not just let people use all the same bar?&quot;  There was no reason why not, so NFPA allowed it.
You're absolutely right; in a lost neutral situation, which happens a lot, current starts flowing across the N-G bond into the earth to the transformer's ground rod and back to the transformer.  However the grounding electrode wires are plenty large enough that the limiting factor will be the conductivity of the earth.