We don't, for some memetic reason, I guess, but many cultures do. New evidence suggest that dogs were actually first domesticated for livestock purposes (but see also this).

Incidentally, returning from the South Pole, Amundsen and his team did slaughter their dogs one at a time, as they had planned to do from the beginning, and used them for feeding both themselves and the remaining dogs. Scott's expedition considered killing their trusty companions immoral (not to mention ungentlemanly), a stance that ultimately cost the lives of both the humans and their dogs.