Not all dogs can even become a good personal protection dog as he is describing, and many of the dogs that take best to such training have higher intelligence and more energy, a bad combination for a lazy owner that just wants a pet when they get home from work. The smarter and more energetic the dog the more attention they require without developing neurotic tendencies. The dumber happy go lucky dogs can receive minimal attention and stay sane, happy, and make great pets. Some people are okay for something in between and like to think they have a really smart dog, just like many parents like to think they have smart children whether true or not, but many really are not suited to taking care of the smarter dogs.
Think of it like mental illness, because that it what it is. The smarter dogs that also have energy need more continuous mental stimulation to not develop mental illness. While some dogs are simply not up to the training from the start and why only some dogs can become police dogs and not all can simply be trained to be one.
Smart working dogs used to get real jobs, like herding animals all day, learning to direct their movements and control a flock or herd both on command and on their own. Not merely obeying but being smart enough to think in terms of risks and managing flocks and thinking independently.
Take that same kind of animal and stick him in a living room or boring back yard all day and they can go crazy and cause a lot of problems. Which is why a lot of dog breeding today actually breeds out such qualities and focuses more on looks than breeding animals based on their merit as working animals. The biggest market is for a docile pet with perfect markings, because the perfect working dog can become a psychotic nutcase if not given a real job that stimulates the mind.
People may think they want such a pet but they are often looking only at the benefits and not the cost in both time and ongoing maintenance of the animal. It is not a machine. It is not like a security system or a gun, and it also can be more of a liability you have less control over too.
Your guard dog gets out and does something and you can quickly be both in legal trouble and financial trouble. A dog bite on a hand for example can get you sued for both immediate injuries, loss of work, even the value of their work for the time they cannot work. Bites a surgeon's hand who cannot work or has a lawyer say they cannot work for a couple years and you may be out more money than your house is even worth.