I’ve Googled this and the answer is nobody knows for sure.
What’s your theory about why as adults, we generally don’t cry out of physical pain unless it’s really bad. Emotional crying I can understand. We try to suck it up.
The last time I cried (unstoppable tears) out of physical pain was about 15 years ago. I fell on my arm and I had a strawberry about 1"x 2", not huge and not deep (though the little bits of gravel in it didn’t help). I was on my way home and it hurt so badly, I started crying in the car. Once I got home and cleaned it, I was fine. I suspect it was the physical pain plus the shock of falling that did it (I tripped because my foot got caught on a low hanging chain I was crossing).