Because the airline won't sell you one if you're on the no-fly list, and if you're on the selectee list, you'll get the magic SSSS tag on it, which will make it that much harder to get into the secure area with Bad Things(TM).

If you're going to have a no-fly/selectee list, then you have to have boarding passes which (mostly) can't be forged, and a way to verify that the person carrying the boarding pass is, in fact, the person named on the boarding pass. TSA has (essentially) done the latter already by (almost) requiring passengers to present an identity card at the checkpoint.

Again, this assumes that the no-fly/SSSS list makes sense ... which is a topic for another thread ...