The VAST majority of violent crime is not committed by highly prepared, practiced assassins. Gangbangers, abusive partners, drunkards... these are the big three

The VAST majority of violent crime is not committed by highly prepared, practiced assassins. Gangbangers, abusive partners, drunkards... these are the big three, and surveillance tends to be fairly good at catching the first and third of those. If you shove someone off the platform on a NYC subway you'll probably be caught and easily identified. The reality is that we witnessed an outlier among outliers, and no realistic system is spending their budget, time, tech or manpower on this kind of thing. The only agencies who regularly do engage in that kind of law enforcement are the FBI/DHS and the Secret Service.