The prerequisites for becoming a cop in the US are almost non-existent. You mostly just need to have a high school degree or GED and to pass a drug test. Then there's something like three months of training, variable by jurisdiction.

At the same time, it's seen as respectable, has good pay and a great pension. Plus the benefit of being seen as an authority figure, which appeals greatly to many of the undereducated.

Like the military, it attracts the lowest common denominator.