It's the DNA that defines almost everything including shape, race patterns, behaviours, hormone levels, skin colour in humans, hair colour tendency in dog breeds. Your statements seem really ignorant with a huge lack of Biology and Veterinary knowledge since every dog breeds have different skills, behaviours, shapes, and IQ's. For example, the "German Shepherd" is the most chosen by Police and Military for some reason:
http://www.akc.org/expert-advice/lifestyle/did-you-know/german-shepherd-canine-cop/
https://thedailyshep.com/why-are-german-shepherds-used-as-police-dogs/

Each dog "breed" has a different "pattern" on the brains and blood that influence their personalities. This doesn't mean they are all individually equal, but the patterns exist and they really define how are a race/breed tend to be:
https://www.outsideonline.com/1857061/20-best-active-dog-breeds
https://academic.oup.com/jhered/article/94/1/81/2187309

The statement you made:

This means pitbulls are not more violent, as the data on dog attacks does not suggest that pitbulls are more likely to bite or attack humans than any other dogs. [...] The reason pitbulls even exist is because human beings bred dogs together, and so the concept of "breeds" is really man-made, as a way for humans to categorize different kinds of dogs by how they look.

That feeling when your "breed" arguments are destroyed (with no wikipedia bullsh*t):
https://www.dogsbite.org/dog-bite-statistics-fatalities-2017.php
https://dogbitelaw.com/dog-bite-statistics/all-dog-bite-statistics
http://www.animals24-7.org/2017/01/03/91-of-dog-attack-disfigurement-victims-in-2016-were-mauled-by-pit-bulls/

Before 2010, pit bulls had never killed or disfigured a combined total of more than 56 adults in one year. From 2010 through 2015, the number of adult pit bull victims increased from 80 to 354, before leveling off in 2016 at 348 (71% of the total number of adult dog attack victims).

Also, Wikipedia references? You ain't skeptic on Wikipedia, don't you? A place where everyone can change it and publish their political agendas...