I'm not saying this to judge the dogs' original owners. In many cases, the dog was feral and there is no original owner.
The important point is that dogs go through developmental milestones just like people and when you get a dog that has already finished its puppyhood, you have lost the opportunity to be present during those milestones and train the dog. Dogs can learn as adults, of course, but correcting bad behaviors is a lot harder. When you get a younger dog (shelter or not), they are more plastic and easier to train.
Yes, of course, it's on you to actually do that training well, but at least you have the chance. When you buy an adult dog, which is what most shelter dogs are, you're kind of stuck with what you get.
The important point is that dogs go through developmental milestones just like people and when you get a dog that has already finished its puppyhood, you have lost the opportunity to be present during those milestones and train the dog. Dogs can learn as adults, of course, but correcting bad behaviors is a lot harder. When you get a younger dog (shelter or not), they are more plastic and easier to train.
Yes, of course, it's on you to actually do that training well, but at least you have the chance. When you buy an adult dog, which is what most shelter dogs are, you're kind of stuck with what you get.