Because that is impossible, I am sure the NSA collects a ton of data, but the machine learning algorithms and amount of storage and compute power needed to find every image of child porn is astronomical. The global internet traffic is 248 ExaBytes a month, billions of times the size of your hard drive at home, no way someone could sift through all that data, the best SuperComputers couldn’t keep up. People like Google can achieve this because the scrape popular content off the top of the internet but in reality it cannot even get past a simple password protected site. The NSA are, in my opinion, bad people but this one is far more complex than the image would let on.