This is true, but it's not the entire truth either. Most congressmen don't have access to the really sensitive intelligence that comes from these surveillance operations, so it can't be that useful to many of them.
A legislator's primary goal is to get reelected, and working to remove powerful operations like this deeply threaten a legislator's political viability during elections. Remember, there's an incredible amount of money changing hands in the military-industrial complex, and money plays a huge role in winning elections.