Why is there no current sharing between neutral and ground?
In your first diagram the answer is no. Not because earth is not a reference, but because there is no current return path to the single phase generator. 

What happens in the first circuit is the top of the generator is effectively "grounded" through the lamp. The bottom of the generator will show the inverted AC voltage. But since there is no circuit... there is no current. Redrawing it like this helps.







simulate this circuit &ndash; Schematic created using CircuitLab

In circuit 2 there is NO current passing to ground. All the current coming from the AC source MUST return to the AC source.

Current sharing would ONLY occur if you grounded both ends on the bottom schematic. Which is what happens with bad wiring, or if there is a short somewhere. Which is why we install GFCIs in circuit breakers these days.







simulate this circuit

Typically though, houses and especially rural farms are wired this way.







simulate this circuit

The service neutral line can be omitted to reduce costs. If it DOES exist, and is rated to carry return current from all phases, the neutral should not be connected to the ground at the client end.

Interestingly your schematic #2 brings up an interesting argument: "Would the electrical system actually be safer WITHOUT all this grounding?" If it were a closed loop isolated system, in order to be shocked, you would need to touch Live AND Neutral.