Lets take the current example of the famous kidnapping of the TV anchor's mother in AZ for example.

If Arizona was blanketed in CCTVs, do you think this kidnapping would have happened?

And if it still did happen, I'm 100% sure the suspects would have been caught by now (11th+ day since the disappearance now).

only if the cameras exist but the perpetrators don't know that they exist. If they know they are being watched on camera it doesn't take a genius to realize you just need to switch cars out of sight. And that is assuming they didn't do that already anyways.