In the case of dogs, when you adopt you ARE buying them. I'm assuming you mean why wouldn't they get one from a shelter instead of a pet store? But they all need homes - shelter or pet store. The issue should be, why do the shelters have to kill dogs that don't get adopted quickly? Well, supposedly they are non-profit and can't afford it, but there are plenty of no-kill shelters and groups that would take these dogs instead of them killing them.
More to the point, many people feel sorry for the shelter dogs but when it comes time to get a dog they want a perfect one. They figure since it comes with a piece of paper that must mean it's a better dog than the ones that don't. Case in point, I adopted a 3 legged dog from a no-kill foster organization. She was on the chopping block at a shelter when they took her in. She was with her foster dad for 1 whole year before I found her. She is the sweetest dog you will ever meet, and honestly I'm glad someone didn't just adopt her out of guilt or pity when they would have much preferred a physically perfect dog. Then she wouldn't have the loving home she deserves here with us! However, she is a pure breed dog, just doesn't have her papers anymore and is one leg short. So she was overlooked, even though she has a sweet disposition and is healthy in all other ways!
For a lot of folks, the breed of dog is some sort of status symbol, or they claim that they have less health problems and are better behaved but that is just their way of explaining why they spend thousands of dollars for that specific dog. Often Pure Bred dogs have more problems then mutts because of too much in-breeding as well as personality quirks. If that's what someone prefers, then more power to them - I say!