Not all shepherds are trained like shepherds
It's more likely you've run into people that have only watched one person work and rather than understanding what they're doing, they just do what they've seen with every dog.
We've had a ton of breeds. Mostly Mal's and Shepherds and really if you want to do the sport you should probably get one of those bred for it. But we've had the usual "offbreeds" like Rotties and Boxers, Am. Bulldogs, pitbulls a Hovawart. Not many ACD's around here. I can't think of any off the top of my head that I've worked with. They do seem to have the attitude for it, but there's a difference in doing schutzhund and really doing schutzhund if you know what I mean.
There's a large leap from getting a dog to play tug with you on a sleeve and a dog that can handle the pressure of a track, OB and then protection with real pressure and conflict resolution.