I think, depending on the area, people tend to not want to get involved due to the fear of retaliation. As a dispatcher, I take calls daily, regarding fights and drug deals. 98% of the time, those folks want to be anonymous, and I respect that. 2% will give their name and number, but they'll ask not for it to be given over the air. They'll usually talk with a cop over the phone if more info. needs to be given, but again, the fear of retaliation is still there.
The fear of having the victim turn on you is also present. If I was walking to my car, saw a male and female in a physical fight, go to the male and pull him off of the female, whats to stop her from turning on me? Now, instead of dealing with one angry person, I now have two.
We live in a sue happy world. A good deed on someones part could lead to issues down the road. I could take someone out of the car in a MVA, but in the process, move the person the wrong way, causing them to never be able to walk again. IMO, I'd rather offer my help by calling the police. I'm not medically trained like they are, so why involve myself in something I'm not capable of doing properly?