As mentioned above, these aircraft are actually less stable at slower speeds, and tend towards stable at higher speeds, so flying a jet capable of great speeds is much easier at those speeds as opposed to reduced speeds. With beyond-line-of-sight secure datalinks, there is really no reason to having manned aircraft in the vicinity. Live-fire exercises with modified, unmanned aircraft (which are not uncommon at present) as targets are not controlled from manned aircraft, but from the ground, and not necessarily from nearby.