We see the end of the ladder is not fixed, as well as being held by the other end by them. Before that, I wonder, how did they toss the ladder to the other end?
Usually, you'd hold the ladder vertically with one end on the ground near the edge, and then let it the other end drop forwards, bridging the gap (provided you've measured the distance and the length of the ladder effectively enough). You kind of hope that the weight/length of the ladder doesn't conspire to snap/bend/weaken it from the downward momentum.
Or two people can hold the middle of the ladder between them and simply toss the ladder across the gap, tethering the near end so the whole lot doesn't disappear into the abyss. There's only so far that you can toss a ladder though.
I seem to remember in the film that there probably wasn't enough headroom in the cavern to do the lift-and-drop technique. So as with many films of this kind, it was placed there
By the power of plot