Because there is some remarkable property there — AIGeometry (you can click on the toolbar Show AI Geometry to see what the hero will walk on). Unfortunately, as far as I remember my last attempts, even erasing the texture to transparent on the original model did not work.

The problem is with the landscape. Align the landscape to the standard (through Zero, I think) at the coordinates of the bridge blocks and check through AI Geometry (each time highlighting the bridge itself) whether the bridge model has been fixed. In general, I had cases where the bridge broke even if the landscape was uneven in another place... In general, test by resetting the cells to the standard value to catch the bugged position.