And very few are killed at pet shops...
Pet shops buy from horribly cruel breeders. Farms really, of dogs kept in disgusting conditions and bred repeatedly on end and then dumped at the shelters or on the streets. I have rescued some of these ten yr old females and they are incredibly sad. Like lifeless dolls who have almost no reaction to anything, cannot play.
I have a new female here now from that kind of situation. She is terrified of being picked up or humans getting too close to her.