Shortly, it is difficult to calm down because panic attacks are not a form of mere excitation/stress, but a form of internal physical agony.
Saying that the difficulty comes from having too much adrenaline to deal with is not a valid explanation. In fact, adrenaline and cortisol can be extremely high in some athletes, and yet they are in full control of their mind and body…
So the difference lies in how the body respond to adrenaline/cortisol, and also in what triggers adrenaline/cortisol release (to avoid confusion, I will only talk about the “what”).
In fact, a stress response is never fully triggered by the brain alone, but by a special arrangement between the brain and what the brain perceives, including the information coming from the internal organs.
Sometimes the stress is self-induced (mainly brain) but sometimes the stress response is a pure survival mechanism in reaction to an external or internal danger. By internal danger, I mean toxicity, organic dysfunction, violent disruption of homeostasis...