This is partly because the three countries have shared cultures, including kanji (Chinese characters), Confucianism, Buddhism, the ritsuryo system (a system of centralized government based on the ritsuryo code), which had been introduced into Japan through countries on the Korean Peninsula since the Kofun period (tumulus period).
それはこれらの三国が古墳時代以来朝鮮半島の国々を経て日本に伝播してきた漢字、儒教、仏教、律令制などの文化を共有したことによる。