The same reason every jackass has to have a Pit Bull.
One family let's their 10 year old child " walk " the dog. The dog pulls the kid where it wants to go. The kid has no control over the dog at all.
A few weeks back the dog was in my yard, barking at my dogs in the back yard. Of course, the Pit Bull had managed to slip loose from the kid's grasp.
I stepped out, told the kid I was going back to get my gun to shoot it. Five seconds later, the kid and the dog were gone.
Having shot and killed another jackass's Pit Bull that had attacked another neighbor's dog, the neighbor's know that I'm capable and willing to shoot their out of control beasts.
Well I have an aggressive breed. But I do keep it for a reason - I don't keep guns, and live in an area where I feel I need it.
My car was broken into twice the two months between the time my Shepherd died and I got my Akitas. The Akitas went beserk at 3 in the morning about a month after I got them (they were only puppies). Turned out someone was trying to break into my car. Haven't had a problem since.
All that said, I have no young children, am well experienced with aggressive breeds, keep the dogs well fenced, on a leash, socialized, etc. etc. etc. And they are good dogs. If I suspected for a second they weren't, they would go.