Hard to untrain an instinct. The dogs are not bread to lick people. They don't take and breed two docile animals and hope the pups might learn to be aggressive.
I was once on a long drive and came across a very large horse in a farm yard. I pulled over to take a brake and admire the animal. The rancher came out and asked if I needed help. I explained what I was up to. We chatted for a spell. I asked him why such large draft horses were gentle giants. He said that if one of them showed any sign of aggression, they would never breed that particular animal, so over many years they developed submissive traits.
Breed two non aggressive pit bulls and their pups will be lovers, Breed two aggressive pits, and now you have dangerous animals. That's how dog breeds get the traits that they have. Selective breeding. Breed two dogs that have the trait that you want, weather it be they are good mousers, good trackers, good pointers etc. I believe they muzzle the dogs so that instinct doesn't overpower training in certain circumstances, (Like the dog gets frightened or feels pain, like a thermometer shoved up their butt). You don't need a lot of encouragement to get most border collies interested in livestock. They know the game right from day one. If they show no interest, you don't breed them with another dog if you want a working herding animal.