I think that's a combination of things. The old Wild West, recent urban expansion and immigration from states with less stringent driving laws.Then again Az does have some of the worst drivers I have ever run across in the US.
Arizona has a history of rejecting regulation, even when it benefits the common good. Urban expansion is a recent thing if you take a long view of history. The section of the 10 Freeway from Avondale past the Deck Park tunnel did not open until August 1990. Arizona is still building freeways right now.
And we've had a lot of people coming from states where they learned to drive on farms or wide open roads. They do not appear to be used to stop signs or red lights. Phoenix is the red light running capitol of the USA. It doesn't help that Arizona's own driving laws also seem to be stuck in the 1960s.
I learned to drive in California, where driving is often mistaken as a right and not a privilege. Urban planning and up to date driving laws tend to make better drivers, although that's just my opinion.
California has the anti-gridlock law where you cannot block intersections or road entrances or exits when stopped. Arizona does not have that. Consequently traffic is every man for himself and signs must be posted if a local government wants to prevent traffic from blocking an intersection.
It took several years after I moved here in 2000 before Arizona finally got with it and changed accident procedures. Up until that point the law stated that at any accident you left your vehicle where it stopped. So, two people with a minor fender bender in the fast lane would be forced by law to remain there until the cops cleared them to move their cars. Great stuff during Monday morning rush hour traffic.