Without confirmation of who is, or isn't, piloting these things, it is quite difficult to answer that question.
There may not be any pilots, or they may be test dummies. Who knows; it is hard to create a credible answer based off the fleeting, often in less than optimal viewing conditions, observations of pilots trying to make sense of an unusual situation at high speed.
I am not saying UFOs don't exist.
But, Occam's Razor applies.
And there is a long and storied tradition of advanced, game-changing weapon systems being mistaken for UFOs, only to be revealed as the newest US aircraft some 10-15 years down the line from the cluster of sightings.
For example, this occurred with the U-2, SR-71 and especially the F/A-117 Stealth Fighter and B-2 Stealth Bomber.
And it is no longer limited to manned aircraft, but also drones of all kinds, and hypersonic missiles.