Nice question. I'll give a example from myself first. I know English so good, I know tons of word, tons of verb. I think thats a bit genetically because there's a lot of English teacher in our family. But when we come to talking part, I am just a trash. Guess, you are in love with a girl. You love her so much, every beatiful thing in your life remembers her to you. You get choice and you will explain your feels to her, but there's a problem. You are shy and non brave person, so that will be harder than others for you. You make a lot of practice, you put all words to your mind and now you are ready. You start with a normal dialogue like "whats up?" Eveything goes perfect, now this is explaining time. But when you start speaking in this part, you can't move your mouth. This is very similar with our English. We know tons of word, but we can't speak. Because we don't know how to speak and pronounce it. All words, paragraphs, stories in lesson books are just teachs word to us. We never make practice. Thats the main reason I guess.