because it is about control and maintaining the current power structure.
a better question is why the administration and politicians like Feinstein lie about its efficacy in stopping "attacks". perhaps their motives lie elsewhere.
This is an incredible amount of information and government does not yet have the capability to sift through it looking to stop crime or terrorism. I'm not even sure that Google, Amazon or WalMart could handle this much data effectively. However, once there is a lead this information can be back-mined to discover links and gather proof. Thus although government could not stop the Boston bombers, they did identify and catch them incredibly quickly.didn't they catch the Boston bombers? Or at least have some fucking clue what those two were up to?
Control and sometimes they let attacks occur so that they can get more control.
Additionally, we should have hundreds of video angles for the Pentagon attack. More than ten years later.... nothing.
There is no magic system that picks only winning lottery numbers or crime about to be committed.
If the government had access to the totally of all communication, and for sake of argument let's say it does, it would not be enough. The reason is resources are limited. People would need to follow up on possible threats and the lower the threshold for possible crimes the more people and money required for the higher number of investigations. It's always been that way. Want to be safe? Have the police stay with you 24/7. Even if it worked no one could afford it. Imagine keeping constant watch on the loudmouths on forums alone. There's no budget big enough and I'm a assuming quantum computers exist and are in use.