Because of this thing called the constitution...maybe you've heard of it. In the constitution, specifically the bill of rights, there's this passage called Amendment 4. Maybe you've heard of it.
The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no warrants shall issue, but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.
So you see...without due process the government can't listen in on my phone or my emails or any of my private business.
Why not just get a warrant from a judge if you suspect terrorism...???