No one is doing WW1 because major part of WW1 was trench warfare.. which would be utterly boring to play. Mainly because trench warfare involved sitting in a trench, and shooting at another trench for months and after these months, noticing that all your shells have hit 200m in front of the trench and repeating the process, of course not without sending thousands of people running over the open and dieing again and again without reason.
While trench warfare was a large part of the war, what most people don't know is that WW1 saw a lot of dynamic engagements that were critical in the outcome of the war. May I suggest those of you who wish to learn more and watch a great movie while your at it to rent or buy "The Lost Battalion" (2001) starring Ricky Shroeder (who puts in a scintillating performance I might add).
There is no doubt in my mind that a WW1 game if done right could be positively brilliant. There are reasons why WW1 by and large has not been a big draw for developers, but they have little to do with the engagements not being dynamic or critical enough.
“The greatness of a nation and its moral progress can be judged by the way its animals are treated.” - Mahatma Gandhi