The Germans were also rather deep inside France, and thus more content to build systems of defensive fortifications. The British and French were of the mindset that the next offensive would have them making territorial gains, and so whichever trenches they currently occupied were only temporary.
Being in occupied territory also meant that the Germans had a huge supply of slave labor from Belgians and Frenchmen, which they weren't shy about using.[1]
Even to the extent that the Entente occupied hostile territory (which was much rarer), they had a much greater respect for human rights. The British, French and Americans had strong liberal democratic traditions, whereas Germany was a highly autocratic and militaristic society verging on totalitarianism by the time the war started.
