I'm one of those still looking for my purpose, and kind of frustrated about the whole thing at the moment, but I guess I'll get over it.
That said, I still think it's possible to put your purpose down in words, but that your purpose will be something more to you than just those words. You may write down a sentence, such as "Live a fulfilling life." That doesn't really say much, but in your mind you'll understand it fully and it will have a deeper meaning, beyond simple language and words.
Secondly, those that have found their purpose, such as Steve, don't seem any worse of for it. In fact, they seem much better of. They know where they are going, they know what they want and they feel good about life, what they do and who they are. I wouldn't mind having a bit of that.
