Why would I thank someone who doesnt exist? Would I thank santa claus for my christmas presents?
I thank the chef if its at a restaurant - they always seem to like that and I thank my host who bought and cooked it if at a private home.
I'd be extremely insulted if someone came to my house and thanked god for the food - god had nothing to do with it, but if they think he did then next time they can come sit at an empty table and wait for him to deliver!