Granted, for some it comes fairly easy, but for most of us who go abroad, it's one of the hardest things we'll ever do. Language is something everyone creates. Everyone constructs his own version of the language. N one speaks it exactly like another person. Those who already speak it are models, but language is not precise, like math.
Language is dynamic.
Since they haven't completely locked in to that sound system, they are able to accommodate another one. It doesn't fit with math, science, and history. It fits with sitting, crawling, standing, walking, running. Adults tend to make language learning an academic pursuit. To them, it is no longer a naturally developed human activity.