Because Apple has their own technical "discussion" site and, while I'm not even sure if actual employees post there, this is a rumor site with a widely used discussion forum not an Apple-supported site. Would you really want Apple to try and get their hand in MacRumors? For one, where do you think rumors that Apple doesn't want out fit in a site with an Apple employee answering questions? Plus, its kind of a useless venture for them. They'd have to create another job for issues that they already have departments to address. It wouldn't be a sound corporate decision and, frankly, it doesn't make much sense.
Plus, how do you know some of us don't work for Apple? 😉