Boredom. Come on, who wants to see the person they're dating all day, everyday? You're stuck with her for 40 hours a week at least. Then you have to see her on the weekends too? When do you get a break? Constant proximity is the quickest way to killing a relationship. So why bother dating a chick you work with? The allure will die out soon enough anyway.
Work place jealousy. What if you moved up in the company and she's stuck filing papers? What if she climbs the professional ladder and you're delivering mail. Either way it goes, work place competition is about as common as six footers in the NBA. Competing with the person you're dating may not work out so well.
Break up. Nothing is guaranteed, especially relationships. If and when this articular hook up fails, you'll be stuck looking at this chick at work. Forty grueling hours a week she'll be right there. There's nothing you can do about it either… unless of course you're getting job offers from other places. Yes, it's better that you never date a coworker in the first place. Save yourself the hassle.