Perhaps it has to do with the fact that in the First war, America was a relatively minor player, not "throwing the hat into the ring" until 1917. Many of the more famous (and costly) battles like Verdun, the Somme offensive, Mons, 1st and 2nd Ypres, had already taken place prior to the arrival of the doughboys. While this takes nothing away from America's sacrifices in WWI, many model manufacturers probably base their figures on some of these major engagements. While some very significant battles were fought after 1917, this would allow them to model the changes in uniform on these figures which occured up to this point. For example, the gradual exchange of soft caps for steel helmets which occured from 1914 to 1916, or the change of French uniform from the colourful sky blue and red pants to the more mundane Horizon Blue seen after 1915. And lest we forget the addition of the ever present gas mask. Let's face it, the doughboys of 1917 looked pretty much the same in 1918. Of course, I stand to be corrected, but it sounds good to me.