Haven’t missionaries already gone to every country in the world?
Missionaries have gone to every country. But, if you visited those countries, you would find multiple languages, religions, cultures and people groups. Just because a missionary went to their country, it doesn’t follow they have all had an opportunity to hear the gospel. In order for this to happen, they need to hear the message in their own languages, couched in their own culture before they can understand it and accept it.