Because these owners don't know how to control or teach these types of dogs. In the right hands they are perfectly fine.Nope. It doesn't logically follow that if something is the result of some factor it will occur 100% of the time.
These dogs were bred for bloodsports, sure owner behaviour, training and environment are other factors but the breed is clearly a huge factor too, any dog can potentially snap but when a bull terrier type dog does it then the consequences can be much worse than with most dog attacks, that's entirely down to the breed.
When was the last time you heard of someone being mauled to death by a Chihuahua?
Yes they were breed for bloodsports etc but how do these dogs get aggressive, they are either taught to or looked after wrongly.
Any medium sized dog can do damage, no point comparing a little thing is it.