Because the one I personaly know, and hang out with, will, without a doupt protect you. Some of them have proven it, and some, judging by thier temperment will. Not all show dogs are sucks. The ones I personaly know, in my group of show friends actually have decent tempements. I never once said all show dobies, but the ones in my circle of friends will.
Jazz didnt have a proper temperment, but she would have and did protect me from harm. But there is a difference between a pet who would protect if needed, and one who could do Schuzhund (sp?) or IPO. I dont think the OP wants a sport dog, just a family pet.
Exactly what did Jazz do to protect you? What did the threat do in response? Again we get down to what the definition is. Bark/growl/show teeth, if a threat goes away, then that is protecting you. What the dog does beyond that is where the question marks lie. Perfect example, a white GSD that clearly did not have the temperament to be a protection dog, yet it's owner was convinced he'd protect, and finally got called out on it by our trainers. The dog would put up a good front, lunging, barking, growling, teeth snapping. As long as the decoy didn't come at the dog and put him in defense mode. Then he'd shut down. They told the guy they didn't want to invest any more time in the dog as he was a liablity because he was a fear biter and lacked what it took. The owner argued, one of the trainers finally said "I'm so confident that your dog will not do anything, I'll let you send him at me and I won't use any gear to protect myself". The owner sent the dog, the trainer rushed at it and yelled, and the dog kept his distance. The trainer then went to the owner and they wrestled around to see what would happen, the dog did nothing but stay 20' away and bark. Finally the dog saw another trainer, and since he was no threat, went after him instead until that trainer yelled and acted aggresive toward the dog. This is how MOST dogs will react. There's nothing wrong with it, it's just how it is.
What you are not understanding is that a dog that will protect you, in the true definition (NOT bark and act scary, but willfully engage and fight a threat until the threat is neutralized) is way beyond what a Sch/IPO dog is trained to do. I'm not taking away from those sports, but to say a dog that does those sports will protect you is not accurate. Sure there are dogs who do those sports who could succeed in protecting you, they are the ones that would succeed in venues that are much more taxing on the dog than Sch. There is little to no defense put on a dog in Sch or IPO. It's all prey work which is fun for the dog. A real life situation is highly defensive and most dogs do not like that. Defense is where the dog tries to get the threat to go away, and if it doesn't that's where fight mode kicks in. Most dogs don't want the fight, and they will go into avoidance and back down. It's a natural response, live another day. Artfish hit the nail on the head with his post.
I know this has gone far off topic from what the OP's question was, but let's at least be clear on what a protection dog really is and make sure that is understood before we start making statements like the show dogs I know will protect you.