It's a good point about geography. The static nature of WW1 means that there are many more shells still in place. WW2 was much more fluid in many places, so not as many explosive devices were left after the war. Still, things like tank rounds are still occasionally found.

Here in Korea there is a popular MTB area where there are fenced off sections with "Danger: Mines" signs hanging, remnants of the war over 60 years ago.