3 Nephi 4:3

And the robbers could not exist save it were in the wilderness, for the want of food; for the Nephites had left their lands desolate, and had gathered their flocks, and their herds, and all their substance, and they were in one body;

The robbers could only live in the wild places because they needed food. The Nephites had left their lands empty. They had gathered all their animals and belongings. They all stayed together in one place.

The robbers couldn’t survive without food, and they had no choice but to stay in the wilderness. The Nephites had evacuated everything—their flocks, herds, and all their supplies. They’d gathered together in one place.