Why do we have different languages?

I've had to ponder the subject of humans speaking mutually unintelligible languages ever since we moved to Germany in August. My husband and I speak German and have both lived here before, twice since we've been married, in fact. He spent two additional years in Germany some time before that. The transition for us is going well. What IS difficult, however, is seeing the kids struggle with learning the language. People always said, "They're kids. They'll pick it up," and "I wouldn't worry too much about it. They're smart." And the like. The fact is, we're really having struggles, particularly with our 3rd grader. He was fluent when he was two and we lived here (or as fluent as a child that age can be). But now it's like he's in a bubble. Nothing seems to really stick in his head. We already nixed the idea of transferring him to a private international school (for the sum of 10K Euro you could get a bachelor's degree, and that's only for one year at this school), and really would like to avoid having our very bright child sent back to 2nd grade. He's loving his innovative math class right now, plus I worry about the boredom factor once we get back to the States. We are both language teachers and are really bewildered at the fact he's not picking up much, despite a lot of extra instruction and input from his teachers and us. The one bright spot this week was tonight, when I sat down with him to read the first chapter of the first Harry Potter in German. He read McGonagall; I read the rest. He's read it on his own several times, and I could tell he was following because he was commenting on specific lines and how they were different in English. (When the teacher heard this plan, BTW, she said, "But I find Harry Potter MUCH too difficult for third graders!" It seems that reading is emphasized much earlier in America than in Germany, and writing, vice-versa. Especially with the Holy Fountain Pen.) Still, it's being a frustrating time for all, and if anyone is reading this and has any ideas whatsoever, I'm all ears.

Back to why we have languages, though. It's a big question for me, even though I love learning them, and am told I'm quite good at it. Why do we have something that divides people so much? Why do people's brains work in this strange way? Why do we fixate on something like accent, even when every word is understandable, every grammar point perfect, and mentally label someone with an accent an "outsider"? Why the divide?

Precise language is so beautiful. Getting the jokes in Don Quixote in the original is great. (Okay, so I've only read four chapters--but they were a funny four chapters.) Wandering through a garden of Rilke poetry is lovely. I'm grateful for the opportunities I've been given to allow me to do that. But the question remains: why do we have to have different languages?