Not sure that's really true.... not by much of a margin anyway. There's a lot of mutts, to be sure... but a large proportion too of apparently purebred dogs.
I'm just going by my own volunteer experience in Pinellas County, FL at the SPCA. About a third are "bully breed mixes" and at least half are Heinze 57's. But out of the ones that seem purebred, how many are from breeders or are from folks that just want some extra $ so they get with someone and breed their dogs? Also, you rarely find small breed dogs in shelters. It is typically the large breed animals with poor canine skills.
"Most of the animals at shelters are street-bred animals, not pedigreed pets."
Not so!! Our dachshund rescue groups check the shelters for the breed. They are fostered by loving people - treated for illness and neglect and placed for adoption to loving families.
Most are pure bred dogs who were not properly house broken or trained and the buyers do not want to bother with them. Some of these are dumped outside the shelters during the night. Some are owner surrendered because they can't afford to keep them - or treat them for medical reasons.
Case in point of a breeder is a 2 yrs old little dachshund who went blind. The breeder dumped her because she is no longer useful for making money!!
This pup is being picked up and fostered until a loving family adopts her. There is even an underground transport where volunteers give their time to transport the pups to other states - and even including Canada. Just out of the goodness of their hearts.
Purebred does not nec. = bred by a hobby breeder. I think the bulk of the purebreds (at least the smaller breeds) in this country today came from puppy mills, mostly in places like MO.
susie