It's not just training it's the fear biting, food aggression, dog aggression and all the other serious behavioral issues that you are more likely to deal with when adopting a rescue dog with an unknown background.
It's one thing to do basic training another to have the equivalent of a mentally unstable convict in your home around your kids. Who the fuck wants to deal with this shit? Not me.
It's like loony women who fall in love with violent criminals because they think they can change them and find damaged goods to be endearing. Like people who think pitbulls are angels without wings and insist they are all good dogs, even after one kills their newborn because someone sneezed nearby.
It's not a dog's fault for being a genetic fuckup or, more likely, having neglectful owners and ending up unsocialized and in a shelter. But it doesn't mean I feel compelled to save these dogs and deal with their baggage, I don't have the time, energy or desire to deal with a fucked up mutt that could attack my kids or do who knows what else.
Support good breeders and support good breeds. I go bird hunting and love doing a lot outdoors so I got a Vizsla and he's been perfect for me and my family. I knew the pros and cons of the breed (allowing for individuality too), did my homework and spent a couple years finding a breeder and waiting until the time was right. So much better than getting some random dog and hoping for the best.