I think that's a big part of it. A driver might have gone by the attempted mugging and even if they noticed it, might have only seen half a second of the incident; not long enough to process it.

While biking to work yesterday, I saw a collared/tagged dog run across an arterial, almost getting hit by a large semi-truck. I stopped and followed the dog. I was eventually led to what turned out to be its house -- it barked at me and was standoffish so I couldn't get to its tags, but eventually the owner came out and the dog snuck back under the fence.

I definitely would not have stopped had I been in my car.