Sometimes people do it because they see the dollars signs of $500+ for a purebred pup (depending on breed). Sometimes they do it out of supposed "love" for the breed. Sometimes just because they can't bother to spay or neuter their dogs and end up with a litter they never planned on (how you cannot know you will end up with pups when you own an unfixed breeding age adult still boggles my mind though).
Actually what makes me mad and has for years is the effect show judges have had on destroying some great breeds. Give the first places to GSD's with very small hips for enough years and gee now we have a whole breed riddled with hip issues. Reward Boston Terriers that have the super boxy heads, now you can hardly find a breeder that will even allow the female to free whelp because the chances are these oversized heads on her pups will not allow a normal birth. Purebreed after purebreed with epilepsy, higher risks of cancer, etc. from far too much inbreeding, the list just goes on. Breeders that breed solely for that perfect show look (and judges that consistently reward that) without real concern for the intelligence, stable temperment, genetic health, etc. of the breed....that what makes me nuts.
Anyway, whatever the reason the fact is people can be very stupid on occasion and there's no way to stop them. Humanity's been doing stupid things for eons of time and will continue to do so. I think it's part of our genes LOL.