Now I don't know why that's the case, but my guess is that that's because when you learn a language as an adult, you almost invariably learn it through your native language, or through your first language.
その理由は分かりませんが 私が思うには 大人として言語を学ぶときは ほぼ必然的に 母語か第一言語を通して 学びます