Because a) they have shown they have zero regard for human life, and b) it would be easier to commit the atrocity than it would be to stage it and get hundreds of witnesses and/or actors on the same page.
>killed 71 people to get to one
>pedophile rings, tests of depravity to prove loyalty
>no way they would brainwash one impressionable Communist, ISIS-supporting outcast to kill 17 kids.
Come on.
OCMC = Overhead Collection Management Center, inside NSA Headquarters