*It’s not really surprising at all.
The type of owners who have the breeds you regularly see are not the sort of people who would value having a poorly behaved and dangerous dog. So will seek to correct undesirable behaviour.
Surprised that as a professional, it hadn’t crossed your mind.*
Many people bring the breeds I see so regularly in to their lives, because they are viewed as "pre-trained" (collie) and nobody realises the high level needs of the dog, that intelligence comes with capacity for overwhelm, and can result in aggression. Medical needs are also a large factor.
Labradors are seen as doting family dogs and are expected to be tolerant of everything. They are also suspectable to things like hip dysplasia.
Cockerpoos are a bit of a mess of a breed known for resource guarding, touch sensitivity, high working drive, but people want them to be fluffy teddies for their children.
There's a lot that feeds in to aggression including genetics, trauma, learning, psychological and emotional wellbeing, and physical health.
I am not denying that there are some people who are attracted to the appearance of muscular breeds but there are a huge amount of factors that play in to aggression - its not all about the human end of the lead.
It has of course crossed my mind many times, however, I am trying to highlight to the OP that she may not worry when she sees a lab or cockerpoo passing on lead as she is with this dog who appears to have no history of concerning behaviour.