They pee in it, so the dirt not only makes mud which would be tracked all around the shelter, but it's heavy, especially when weighted down with pee.
The wood chips / sawdust / shavings are lightweight, absorb the pee, and easy to shovel up into garbage bags for disposal. Plus, big plastic bags of these wood shavings can be stored at the pre-disaster warehouses, and can be stored at the shelters themselves when they are setup. Think of the big bales of wood shavings sold at pet and feed supply stores for lining cages or dog beds. That's the kind of thing I'm talking about. Very easy to work with, both before and after being "used."
What was I thinking?
Why does't anybody make 'scoopable' doggy litter?
I have a bale of those shavings that I use for my bunnies, geese and goat.
Wonder if I could convince the dogs to do everything on a pile of them placed out in the yard?
[I know, I know...I'm a dreamer]...;)