Why do people generally buy costly pure-breeds over free mix-breeds?

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I work at a Animal Hospital and I've seen more pure-breed dogs come in with various health problems then I have seen mix-breeds. I've only owned mix-breeds and I've found them to be intelligent and just as loving and cute. There are so many homeless mix-breeds that are stray and do not get a chance at a loving home where as you hardly see homeless pure-breeds. What is your opinion about this?