Stress is a natural human response to certain stimuli, programmed into our biology as a survival mechanism. That said, in our society, that stress response is out of control. We've invented more and more reasons to trip the fight or flight response, to the point where we pretty much always have a background hum of anxiety.
You need look no further than breathing rates to realize this - most of us don't even remember how to properly breathe, full and deep from the abdomen. All humans breathe like this as infants, but as time goes on, tension builds in our bodies, we gradually forget this technique and we start taking shallow, chest-centered breaths that only use a portion of our lung capacity. The average breath rate is 15-20 breaths per minute, which is in reality quite fast.
This lack of oxygenation makes it difficult for the body to shut down the stress response of the sympathetic nervous system. The weaker breathing essentially depletes the parasympathetic nervous system, which is what shuts down the sympathetic. As such, we keep on straining our bodies, with what is essentially a never-ending stress response. As I said earlier, a background hum of anxiety.
To learn how to reinforce the parasympathetic nervous system and essentially eliminate the stress response on stage, have a look at the following video:
I hope this helped, and please feel free to ask any followup questions you may have. Thanks for reading/watching!