Because that only allows for you to sight in at that range.
Unless that range also exactly matches the range to the enemy targets, from your current nest, you won't be properly sighted in for your intended targets.
The ranges over which snipers make kills is sufficient that even a very small miss-alignment of the sights translates to a miss (often by many yards) at the target. According to Wikipedia [1], the sniper record is 3,540m (3,871 yd). That is 3.5km - imagine targeting something 3.5km away, firing, and having your shot hit the target.
Even in 1916, while sniper's may not have operated at 3.5km distances, they could easily have been operating across 500-1000m distances (the Wikipedia article lists a 1918 record of 1,280m (1,400 yd) or 1.2km). Even at those relatively shorter ranges, the margin for misalignment is still tiny. You have to actually sight the rifle in for the actual range, the actual elevation (whether you are shooting uphill or downhill) the current wind conditions, etc. And still hit a rather small target. Aligning to some other fixed berm will not be nearly accurate enough.
Unless that range also exactly matches the range to the enemy targets, from your current nest, you won't be properly sighted in for your intended targets.
The ranges over which snipers make kills is sufficient that even a very small miss-alignment of the sights translates to a miss (often by many yards) at the target. According to Wikipedia [1], the sniper record is 3,540m (3,871 yd). That is 3.5km - imagine targeting something 3.5km away, firing, and having your shot hit the target.
Even in 1916, while sniper's may not have operated at 3.5km distances, they could easily have been operating across 500-1000m distances (the Wikipedia article lists a 1918 record of 1,280m (1,400 yd) or 1.2km). Even at those relatively shorter ranges, the margin for misalignment is still tiny. You have to actually sight the rifle in for the actual range, the actual elevation (whether you are shooting uphill or downhill) the current wind conditions, etc. And still hit a rather small target. Aligning to some other fixed berm will not be nearly accurate enough.
[1] https://en.wikipedia.org/wiki/Longest_recorded_sniper_kills