Why can't I put a load between hot and the ground line?
Because the grounding system is a safety system, and is designed to only carry current during a fault.  If you use a grounding conductor as a grounded (neutral) conductor, you'll have current flowing on water pipes, faucets, appliance frames, metal electrical enclosures, and anything else that's grounded.

Doing this could lead to property damage, injury, and/or death.