It is also about weight, jaw size, bite pressure and propensity to hang on.
Those are all physical characteristics of breeds.
An aggressive schnauzer with a crap owner would never be able to do the damage that a bully or even Staffie (still a BULL terrier) could do.
They were, as a poster pointed out, bred to be aggressive, fearless, risk taking and "bonkers" enough to attack bulls, they were bred to go for the bull's sensitive and fatal areas (like it's neck) and to be extremely tenacious at hanging on.
It is the breed.
There will always be bad owners.
The breed they own, however, dictates the chances of survival/serious injuries.