That's my point, actually. People tend to assume that anything they do is being watched, where the truth is that whether or not a given item is stored depends on whether someone has specified it's interesting somehow, or whether a trained program has.

They don't store everything in case it's needed, nor do they have programs capable of detecting everything people suspect is monitored.

It's still an invasion of privacy and probably breaks various laws, but it's not the complete surveillance people tend to think of, and it's not as useful as you'd expect given the money invested and the effort put in to it.