why is this?

You need your stomach to be empty when you go under anesthesia because one of the side effects is vomiting. Since you can't move, you can aspirate (inhale) the vomitus into your lungs, which can cause really serious problems.

because anesthesia can cause relaxation of the lower esophageal sphincter resulting in the food contents that could be in your stomach to come up, with the possibility of aspiration (substances other than air entering your lungs) occurring. this can be detrimental, result in infection, or even suffocation. it is just a necessary precaution. (I'm sure you've read somewhere about how a drunk person died b/c they choked on their vomit, its the same concept).

Because the anesthesia can cause you to vomit your food. When you are under anesthesia, you're not able to keep your throat clear by coughing or sneezing and you may wind up choking on your vomit.

I'm not sure, so don't take my opinion as a proffesional one, but it's probably so nothing in the food mixes with anything in the medicine or shot they give you. It's prob. just a safety precaution.

Because anesthesia makes you nauseated. If you throw up while under, you could breathe the vomit down into your lungs and die.

Nothing except water and maybe black coffee no food unless you doctor advises otherwise.

Because it might make you puke and then it could go into your lungs.

Puke on yourself in your sleep