Because panic attacks get our body's "defence mode" started, so it gets in the fight-or-fly attitude. This "mode" involves many organs and glands, and in general terms it rushes your body's rythm. As a result, your inner temperature rises and your body sweats to cool off.
The thoughts and feelings are overwhelming. It is a human response to threat... fight-flight-freeze. Perceiving a situation as threat would increase heart-rates, breathing, adrenaline to response quickly, either run or fight it. But as these physical response get overwhelming panic attack occur. Physical sensation becomes overpowering and amplified with the thoughts that "I cannot handle it" or "oh my goooosh, this is too much"..etc. Then panic attack is there, and sweat is a way for body to cooling the core muscle for a better response. In common, sweaty hands when encountering a challenging or fearful situation... eg: public speaking, video interview... are very common. Learn to notice the physical sensation, notice the thoughts and feelings, but not let them overpower you; could be a very good way to handle and work with your fear. Therefore, panic attack may not need to occur since the way you handle anxiety is different... accepting, breathing into it, noticing anxiety, normalize anxiety... may actually long term work very well along with what you really want to do in your life.