I think human ignorance.
You take a GSD six month old puppy. If it started barking and growling and nipping at it's owner, the owner would realize he/she has a problem and take steps to correct the behavior. They would take the behavior seriously.
You take a Chi six month old puppy. It exhibits the exact same behavior as I mentioned. The owner thinks it's funny. "Look how brave my little boy is! He thinks he's a big dog!" and the owner will continue to encourage this behavior by showing off to all of his/her friends the behavior.
Hondo Von Dopplet L Bauernhof "Hondo"- GSD
Lilie's Tug McGraw "Tug" - Golden Retriever
Maggie - Mini Dachshund (Rescue)
Lonestar Pivo - Texas Blue Lacy
Ashe - Barn Cat
Katie / APHA