Good grief, are you trying to be obtuse? SA mastitis is just one good example of the prevalence of Staphylococcus aureus in cattle. There are more.
Who gives a rats behind how it was introduced or whether “it’s part of bovine normal flora”. SA in cattle is not “a rarity” as you proclaim. SA IS contagious meaning it can transfer from on animal to another. Once it is introduced it can proliferate without human intervention. If it exists at all in a given animal and that animal is overtreated with antibiotics for whatever affliction then there a more than fair chance that a resistant variety will emerge.
The point is the origin of SA is humans, not animals. Furthermore, antibiotics are also overused in humans. How many bacteria do you think developed resistance to Penicillin, long before it was used in animals. It is the nature of the beast. As long as we use antibiotics, resistant populations will emerge.
It is estimated that a new viral disease emerges somewhere on the planet every 3 months and this has nothing to do with antibiotic use.