According to a friend who works for the water company, there's 2 reasons:
1. We have a tendency to pack the dirt tighter has the hole begins to fill back up again, and
2. Because we almost never got all the dirt we removed from the hole back into it. Especially is you simply shoveled the dug up dirt, onto the dirt surface around the hole.
He also taught me a little trick: Shovel the dug up dirt onto a tarp placed on the ground next to the hole. In that way ALL of the dirt removed, goes back into the hole, that's how they, the local water company, does it.
