Since when do American songs "tell what they are all about"? If you're really that curious, search for translated lyrics, or check out the Jpop that's at least partially in English. Pop is pop, and often there's not much difference in subject matter, even if there seems to be no real meaning, just words that "fit". Though as far as lyrics go, I'd compare Jpop to early 80s Britpop: very similar, really.