Have you ever spend 30 minutes searching for a user interface element or menu item that you knew must certainly be there, only to eventually learn that it had disappeared completely because you were in some state where it couldn't be used?
Imagine that you don't know that you can't vote for your own post, and you have decided to upvote your own post. Or imagine that you recently learned about upvoting, and you have decided to upvote all the good posts about pie that you see to help promote their visibility, and now you are looking at a post about pie which you wrote but which you forgot you wrote. In any case, you're looking for the UP button.
You might spend an inordinate amount of time looking for the upvote/downvote button; you might even go so far as to waste someone else's time by asking a technical support person (or posting in meta) saying "Where is the upvote button?"
It is in my opinion a good heuristic not to make things disappear completely simply because the current state of the world is such that that thing cannot be used right now. It is usually better to leave the button there, possibly in a visually different state, and allow the user to try to click it and then have an opportunity to teach them why it can't be used.