As the level increases, the technicality increases. Eventing is not only about the height of a fence: 5* is only 1.20m xc. Unlike SJ, the terrain is also highly influential. It takes time and training to build up through the levels.
A major changes is the speed across country, which obviously means a combination has to be more adjustable, more balanced and quicker in reaction times as they move up the levels, which generally means more experience.
The technicality moves up too. It is logical. At the lowest level the water jump may just be to pass between the flags through the water. Then an obstacle may be placed a few meters before the water and the jump becomes elements A&B because when the horse sees the water, it will influence how the horse jumps that first element. Then moving on up, the A element is placed closer to the water, even to jumping over and directly into the water - which is far harder to do. Then another obstacle is placed after the splashy bit and becomes element C because riding through water will affect the stride into that third element. Getting harder, place the third element ever closer to the water. Harder yet, introduce an obstacle in the water, to jump out of and into water and the problem is now becoming seriously challenging. Then the several parts all strung together, at increasing speed… Then throw in curves, slopes, light and shade to add to the technicality.
A course that included an 80cm obstacle jumped directly into water, onto an 80 cm obstacle set in the water, onto another 80cm obstacle set one or two strides beyond the wet bit would be highly technical and I personally would give odds that no combination would make it through. At 80cm, many combinations don’t even make it between two flags on either side of a pond.