Are they? They are certainly rare, but so are actual terrorists attacks in the United States. If you choose at random an attack on the US outside of a combat zone within our lifetimes, you have a nontrivial probability that it was an admitted false flag operation. And that's just counting the ones they've admitted.
This isn't to say that we should fall in with the "9/11 was an inside job" people, but rather that we should be extraordinarily skeptical when there is some kind of an attack and self-interested parties come out of the woodwork with complicated one-sided proposals to ensure "never again" (even though they can ensure no such thing), which just so happen to secure to the proponents an unfathomably large quantity of tax dollars.