Because one can never know anything, to know anything one must be certain of something, one must follow natural laws which he must be certain of, but to be certain of a law one must understand that said law will be different in different circumstances. Therefore, to be certain of everything, you must test it under all circumstances and thus must test it an infinite amount of times, requiring an infinite amount of time, which you don't have. Therefore, I knew you did know of that knowledge for that knowledge cannot be known by known people in knowing.
What's your favourite television program?