How come anxiety produces physical symptoms?

It’s been months but I think I’m starting to make the connection that my generalized anxiety disorder is making it harder for me to sleep. On nights where I’m not nigh paralyzed with terror at bedtime, I sleep better and have less aches and pains during the night, and upon waking up.

I absolutely know my anxiety contributes to my gastrointestinal symptoms.

Yes I have medication to be taken as needed (propranolol), but I want to avoid overmedicating myself or medicating myself unnecessarily. I usually just take one pill before my work shifts, and never outside of a work setting.

Anyway, but why? Why would anxiety manifest as physical symptoms instead of only mental/emotional? I thought it’s all just in my head.