There's just too much volume to track all the content everywhere.
There are 350 million people in the USA, more or less. Including kids not of age to use computers. One computer, just one, operates at billions of instructions per second (when the code is written in anything efficient, like c.) The NSA has a newish huge data center located on the main trunks.
You do the math. If you still think they can't sieve that amount of data effectively, why then, good on you for your optimism.