Because chocolate isn't fatal to dogs. I'm somewhat old, and the whole "chocolate will kill a dog" thing has come along fairly recently in my dog owning life. The last decade or so I guess? When I was a kid I had a dog who was passionately fond of brownies, and I can't even guess as to how many pans of brownies were lost to us because he managed to get his paws on them. The only way to keep him out of them was to put them in the fridge, and countless pans of cooling brownies that were set where we were sure he couldn't reach them.....well....they became his.
He was likewise hooked on chocolate milk, candy bars...anything chocolate.He not only lived to a ripe old age but he never even got fat! (Highly unfair in my opinion. I get fat on chocolate.)
As I got older I had other dogs who adored chocolate and I always shared a little with them. There were other instances of chocolate thievery and honestly I never even had a dog get sick from chocolate. It never seemed to harm them a bit.
Then lo! Suddenly chocolate is lethal to dogs. I can tell you honestly that when I heard that my eyebrow was about six feet above my head while I thought of all the dogs who should have perished in my lifetime, most particularly Joe, the brownie fiend.
So, I asked my vet, who told me that chocolate contains a lot of caffeine and SOME dogs are highly sensitive to caffiene and can suffer fatal arrhythmia's if they consume caffeine. Since there's no particular way to know if your dog is or is not sensitive to caffeine the most sensible recommendation is to tell people not to give their dogs caffeine. Since the caffeine containing food dogs are most likely to want to consume is chocolate.....don't give your dog chocolate.
I've had other vets tell me there are other substances in chocolate besides caffeine that dogs can be sensitive to, so it's best to avoid it on general principle.
As of now I live with an 11 year old dachshund who has stolen a fair amount of chocolate in her lifetime, as well as a 7 year old poodle who has done the same and are both fine.
I have a lab/border collie mix who is a goat and eats things that aren't even edible, so I find it hard to believe chocolate would hurt her if styrofoam, wood, plastic, fabric and paper don't.
I'm firmly convinced, though, that it would completely kill my pit mix boy, because he's so food intolerant.
I don't give it to any of them though, because why take chances?
But that's why your dog lived....and why mine have.