Why don't co-workers put stuff in my in-box?

I have a very clearly marked in-box, on my desk, on the edge of my desk for easy access. So why do co-workers continually ignore it? They place items on my desk, or on my chair, if they do use the in-box they don't put it in neatly, they lay it across the top or only put it in half way with the ends sticking out, usually over the edge closest to my trash can! Why don't they put stuff directly in my in-box? Do others have this same work issue? How would you go about solving it? Thank you for all sincere answers. Have a great Friday!