If you want the basic answer, AOE DE is a remake of AOE:ROR and AOE1. And AOE:ROR, which had that mission, is an add-on/expansion to AOE1, which was basically a clash of civilizations game that led up to the Roman times. So the basic idea of AOE1 was civilizations like Greece v. Egypt, with Rome being more of an afterthought epilogue (although quite long actually). Germans and Celts were neither an empire like Greece nor were they a player in the time frame AOE1 was developed for.
More in strategic terms, the problem with making the Gauls and Germans as civilizations is that they were not really on the same par in terms of civilization as Rome until like the 5th century when the AOE2 game describes events. I don’t know if you are familiar with the Roman suppression of Boudica’s revolt, but the Roman forces (just one or two legions) wiped out far larger numbers of Briton warriors and civilians, sadly. There was a famous battle where Boudica had far more soldiers and got killed.
You can make the argument I think that for aesthetic purposes you would want to have a barbarian set of architecture for these missions, and I sympathize with you. You are right that the Chinese architecture doesn’t really do it here. Plus, someone can also make the argument that there was some kind of Western European non-Roman “civilization” in Roman times. It’s just that in terms of power they are more relegated to the AOE2 medieval period.
If they make a separate Barbarian Civ, then people will want to play it and maybe be disappointed if they don’t find it very balanced against Rome. Historically, the Barbarians were not a match for Rome, generally speaking, until the AOE2 period. I don’t want to overstate the case though. I believe that the Celts conquered northern Italy or even Rome itself at one point before Caesar’s time. And Rome made a strategic decision not to conquer all Germania after they got some defeats there.