Because it's an icky mess with everything.
Wet, muddy animals, muddy truck inside and out.
Getting self wet, cold, and already being run down myself, it's a recipe for getting sick.
It also tears my roads up driving on them when wet and mushy, and I'm the one who has to fix them.
Part of having good roads is staying off of them in bad weather when possible.

God Bless,
David B