S. was aghast. "Boys are crazy" she said, "Girls would never do this kind of thing". She was also kind of surprised that boys would go to such effort to build structures which must have taken much time and organization to complete. When the kids play capture the flag and it is boys against girls, the girls always win. She explained that the girls make plans while the boys just make their plans as they play.
A couple of thoughts began to percolate in my mind, but it took a while for them to fully form.
Boys are not fearless and they really do not like to plan everything out ahead of time, but they can ease their way into both things. The way I envision it is that they started with a small jump that could be made with a fallen log and some rocks and dirt. Now they were used to making things and the thrill of a small jump, so they progressively made bigger and more elaborate jumps and became inured to the risks gradually as jumps became more elaborate. We just saw the end of a long learning curve--they just didn't decide out the the blue to spend weeks building a suicidally high jump and then have the nerve to use it. They got there in a gradual and natural way.