Because there's a limited number of ways that people do and say things. If you have a pattern to look for, it becomes easy for the computer to find the needle in the haystack. I would imagine they are doing some obvious searches like people buying specific things, going specific places or contacting specific people.
sure, I can see being able to put together patterns that say - oh, if a guy named ahmed buys gasoline, fertilizer and rents a uhaul, we got a problem.
But actually information out of huge data sets (not just an individual case) is really fricking hard. even for the NSA. This doesn't mean there is no risk to privacy, there is. I suspect they are collecting all of this information knowing computing processes are accelerating and technology for sifting large amounts of data is improving.
I would love to be the storage vendor for the NSA, I bet they spend a shite ton of money.