I think it depends on who you are specifically referring to.
If you are referring to bonafide breeders who take the time and effort to do what they can to ensure a reduction in congenital and other health issues, and follow proper health care guidelines, it is likely because they know just how intensive proper breeding is... and how unpleasant and costly it can be to do it right. Many of these people also rent / lease / donate kennel space to rescues, or have their share of rescues incorporated into the home. I don't know a single breeder of working dogs that I find respectful, for instance, that does not have broad experience in animal rescue.
Then of course you have other camps, such as those who sell pups out of boxes for $25. If you hear it from them, I'd guess it's an effort to make them appear to care more about the animals than say, they do about raising a few bucks for tickets to the next lawn tractor race, or a 24 pack of Pabst. Or perhaps, this was a bonafide accidental litter and they have learned a lesson, and are trying to pass it on...