They probably believe that they will not be targeted by it, that if an algorithm scans over their data that it's no big deal, and that it may lead to some criminals being caught. They can live their entire lives from start to finish, and whether their own communications were ever intercepted will likely have had zero impact on their life.
I'm not trying to be dismissive of people who are concerned, just wanting to point out that there are also valid reasons to take the other view.