The bag will rip and tear over time because the contents are shifting independently of each other. This will not happen (as quickly) with a non-Newtonian liquid

The bag will rip and tear over time because the contents are shifting independently of each other. This will not happen (as quickly) with a non-Newtonian liquid. Not to mention, dirt and gravel roads aren't big bags. The materials are simply laid out on the ground. They also don't last very long under high traffic or heavy loads.