Why not? Because dogs are smart and other animals are not? Pigs are pretty smart, you can house-train them and teach them tricks, and there are chickens that scientists have taught to play tic-tac-toe. If the morality of eating a certain animal varies with it’s intelligence then why don’t we eat more snails and slugs? Maybe it’s because puppies are cute, but most animals are cute when they are babies. Ok, there is that man’s best friend rumor, the notion that dogs were raised to be our companions, but pretty much every animal you slaughter for its meat is a “companion”. Modern cattle and poultry are too soft to survive on their own for very long. We have bred every edible animal into a state of weak, stupid dependence on us. 

My point is that there is no reason beyond the obvious cultural taboo, that you should be unwilling to stew the family pet Labrador tonight. The main, motivating factor behind that is that other people would think us strange or sick for doing it, also, because it is unfamiliar, a different idea. You would be ostracized among friends and family, you could be arrested, even if it was not illegal, they would charge you with not having a license or something like that, but they would take you to jail before they decided that what you did was wrong. All because it is not customary here to eat certain animals. 

Suggest the idea and you will be dismissed not on any tangible, thought-out notion of whether it would be worth the trouble and expense, but because people are afraid of being different.  Nobody questions whether the way they live their lives makes sense, they just do it because everybody else does it, so, supposedly, it is the right thing. They go to school and study the thing that seems to make sense for everybody else, they marry some the people they know approve of, or somebody like their mom or dad, they have kids, not because of any necessity to have kids but because of a group-compulsion to conform and breed. Their friends are doing it, people their age are doing it and have always done it. Then a few years later they get a divorce and squabble with their messed up kids just like everybody else is doing. 

Human life is a rut carved out of the mud by the unhappy, bitter generations before yours, and you are walking right where you are supposed to walk. For those who decide to rebel, it is merely a matter of switching ruts. Everybody is somebody else’s loser. 

In some places they would think you were crazy for letting a dog get old and too tough to chew properly.