Why doesn't current return via the grounding conductor when it's connected in the panel?
Yes and No.

There are two separate systems with "ground wires" in the typical US residential service, the three wire Edison service.  

The Grounding Electrode System (GES) consists of ground rods and other grounding electrodes, connected (bonded) by a grounding electrode conductor (GEC) to the neutral at the first means of disconnect.  

The Equiment Grounding System (EGS) consists of the equipment ground conductors (EGC's or ground wires) run along with all the feeders and branch circuits in the house.  The EGS is also bonded to the neutral at the main bond, nowhere else.  

Under normal circumstances, no current flows on the EGS;  it does not provide a path back to the neutral at the panel to complete the circuit.  Current flows on the EGC when there's a ground fault so that the fault will draw enough current to open the fuse or breaker, clearing the fault.  

The GES doesn't carry much current under normal circumstances, but whenever there is current on the neutral, there is some current flowing through the GES, through the ground, through the neutral bond to the ground at the utility pole.  

Current doesn't only take the path of least resistance;  it takes all paths, with more flowing on low resistance paths, and less flowing on high resistance paths.  

Normally the neutral wire has very very little resistance, and the ground has significant resistance, so although they are parallel paths back to the utility transformer, very little current flows in the ground, almost all flows through the service neutral conductor.