I can see why this might seem like the case with present time flying wing aircrafts. But most of the initial design and theory for the flying wing design (such as the Horten Brothers) was actually done on a glider which is passively stable. Most, if not all, military fighting aircraft (like the F and B series) are purposely designed to be dynamically unstable since this allows much faster control and reaction time at the expense of special computer and ASICs to maintain aircraft stability.
