Well, technically speaking, there are two reasons:
1. in many parts of the world it is frowned upon or even illegal to trade dog hide aka "temple fox"
2. if you take a life sample you have to take care of mites and other parasites that might devastate your supply of organic tying materials.
I just received a few handfull of brown labrador underfur from a friend which is an excellent seatrout fly dubbing. It has spend a fortnight in the freezer and is now good to use.
Actually a lot of my mop jigs, have collars of pure white Alaskan husky fur. Never saw a problem with parasites as these pups are extremely well cared for by neighbors across the street -when they brush out the loose fur, I get bags full of the beautiful, free flowing, soft hair !