Because they want to and have a sense of entitlement tbh. Plus, an awful loot are naive and do not understand about or know just how many dogs are being put to sleep every single year.
I am a 'breeder'. Golden Retrievers. I admit I do it for selfish reasons, my dogs are my pets, but I adore the breed I have chosen to own and would like to think that by breeding them I am doing my own little bit to improve the breed. I also like showing them and you can't do that with non-pedigree dogs.
I am sadly only too aware of the rescue situation. It would be very naive of me to think I could never contribute to that as obviously one my puppies have gone to new homes then it is out of my control. However, I do my absolute utmost to ensure the puppies I have bred do not end up in rescue buyers signing contracts saying they must return puppies/dogs to me if they can no longer keep them, maintaining a good relationship with buyers, throughly vetting and choosing what I consider the best homes etc) and none have so far. I also only breed very infrequently, once every 2-6 years on average I suppose and when I want a puppy to keep and show. I always have a waiting list prior to deciding to breed, but I certainly don't breed on a 'supply and demand' basis, I breed for myself. I am extremely fussy about where my puppies go.
In my case, it would be unlikely that puppies were left unsold because of waiting lists. If they were, the answer is simple, I would have to keep them. However, in the case of those breeding without proper thought, a lot end up being handed to rescue or PTS. (either humanely or otherwise
)
I can see why people disagree with breeding at all when there are so many dogs/cats in rescue, but very few of these come from responsible breeders, but originate from puppy farmers and BYB's. I think the key to solving the rescue situation is much tighter legislation surrounding dog breeding and lots and lots of education to the dog owning public about how to source a dog or puppy responsibly. Sadly though, some people feel it is their right to own a dog or cat, whether their home circumstances are suitable or not and these people will be turned down by rescue and responsible breeders, so they then decide ot use a BYB or puppy farmer. 