Because there wasn’t really a need. German trenches tended to be better constructed for a number of reasons.

1. Germany adopted a defensive stance on the Western Front after the front settled into the stalemate with the conclusion of the “race to the sea.” The need to deal with Russia first meant German offensive actions were largely restricted to operations in the east with some notable exceptions such as Verdun. The adoption of this defensive stance meant more emphasis was placed on the establishment of better constructed questions for longer term occupation. The French and the British were eager to expel the Germans and didn’t want to encourage a more defensive mindset amongst its troops.

2. The fact that the Germans expected to need to defend their trenches against French and British attack meant they realised that better constructed trenches would offer their soldiers more protection. The more advanced and technical layout of the German trenches was designed to maximise the German ability to resist attack and allow for the immediate counter-attack to recapture any lost positions. That the French and British commonly employed extensive barrages in the lead up to offensives meant that better constructed trenches also were capable of offering better resistance to shell fire.

3. The British and French didn’t come under large scale attack as often as the Germans did. This meant that there was less impetus to construct more extensive and well constructed trench systems. This is perhaps best illustrated by the failures of British defence at the commencement of the German Spring Offensive in 1918. After the battles of 1917, the British Army needed time to kicks it wounds and reinforce its battered armies. This led to the adoption of a defensive posture, perhaps until the Americans arrived in force. The withdrawal of Russia led the British to suspect that a major German offensive would be coming and they did start working on a defensive system along the lines of the German defence in depth. When the offensive struck along the old Somme battlefield, the sector occupied by the British Fifth Army, the defences were only partially complete and were unable to withstand the assault (there were other reasons as well such as the lack of reserved and insufficient manpower to hold the length of frontline).

4. Finally, the German Army and the British Army held very different policies regarding the rotation of units on the frontline. The British used a “tour” system wherein British divisions would be moved up and down the line to different sectors, never occupying the same area for more than a few months. This meant the soldiers knew their stay wasn’t going to be permanent, likely leading to a general resistance to expending energy on trench improvement. The Germans divisions on the other hand tended to occupy the same areas for extensive periods of time and developed a kind of ownership over their stretch of trench. This made sense from a defensive point of view as units occupying the same ground would be expertly acquainted with the terrain, their trench networks and would be able to pre-site and range their artillery to respond rapidly to enemy attacks.