Well fact is, that it sells a lot better if the US is in the game, and if the US is not the enemy.
Look at Master and Commander... They made US to France because they knew that people in the US won't watch it otherwise.
And then BF1, added US into the game before France, because of the US audience. Even though the US played a lesser role than any other Nation during the war. BF1 is crap anyway, not really historically accurate in any way, but its still retarded.
And yeah sure, France didn't play any real major role during the WW2, but come on, give us something else. We always play as US or Soviets, give us French or British as playable factions. Or... god forbid... A campaign as Germans.