Perhaps because they get awful darn tired of having to kill his unwanted progeny?
Spay or neuter your dogs! there are lots and lots of breeders out there. Too many. Not everyone needs to be one. The number of healthy but unwanted pets we put down in this country every day is a travesty.
And the next time you consider how cute it would be to have a litter of pups, go down and just spend a day at the pound. Count the ones with the red tags indicating their time is up. If that doesn't convince you, I don't know what will. The homes you find for your pups could have easily taken home one of those poor souls.