For much the same reason you shouldn’t drink the water, you should not eat directly off the tray table in front of you. In fact, you shouldn’t touch it at all unless you wipe it down thoroughly with a germ-killing cleaner such as Lysol (which you can bring in wipes form on your trip). Airplanes are turned over so quickly, getting ready for the next flight and set of passengers, the tray tables are never thoroughly cleaned and disinfected. All manner of disease carrying germs can be found on the small piece of pull-down plastic in front of you. If you were to look at it under a microscope, you probably wouldn’t even want to sit near it, let alone eat off it, or touch it at all. Do yourself a favor, pack some disinfectant wipes. They’re also useful when you use the restroom!