Incorrect dogs or bad handlers?
I'm not sure to be honest, I guess it really comes down to the individual dogs & handlers.
I am a novice handler and my dog is reactive with other dogs. I'm sure there are things I need to improve upon as a handler (scratch that, I *know* there are), but I socialized the heck out of my dog.
This was a problem that was evident very early on, I'd say 9 weeks of age or so? I'm not sure if the breeder would have known about this, because the puppies were introduced to her closed pack of dogs, they hadn't had all their shots so were probably not introduced to dogs outside the pack. She did take them around to grocery stores etc. so got lots of human socialization before she came home with me.
I had her in puppy class at 10 weeks of age, grade 1 overlapped the last week of puppy class and put her into grade 2 a couple of weeks after grade 1. I took her out to flyball tournaments to see all the dogs in action, scent detection workshops, I took her to schutzhund training to watch, I take her on hikes with my friend and her 4 dogs. She was in doggy daycare for months and months, I took her to the park, by the splash pad to see kids running around and playing almost every night for several weeks, we sat outside the grocery store to see people come and go, I even took her to Bass Pro Shop and put her in a shopping cart and browsed around.
So while I do think I could be a better handler, I also think I worked pretty darned hard at socializing her.
Even her breeder told me, when I asked her what I was doing wrong, that it shouldn't be so hard. So while I may have some failings as a handler, I also think my dog is hardwired to be this way. I keep working at it, but I honestly am not sure if it's something we can overcome. I hope so though.
As for dogs in shelters, they have often come from abusive or neglectful homes and can be fearful at first, until they learn to trust again. I don't know if you can ever really get a true sense of what a dog really is at a shelter, I think they are probably scared and in survival mode.