It's also a clarification between "I would like to, but no thanks due to these other circumstances, maybe another time" vs "No, would not like to, probably not ever." It's also giving you the opportunity to counter-offer, as it were: if you're not particularly hungry either, so maybe something more like a small appetizer and a drink?
Plus a million other options, of course.
It's not really a yes or no question to begin with, even if you're thinking of it that way.
One time we were discussing an upcoming trip and she mentions that her friend invited her to come visit another city. How she phrased it made me think she was inviting me on the trip, even though I wasn't really interested, but she made it clear she wasn't. lol. Sometimes I have to sit her down and tell her that I don't mean anything by asking direct awkward questions because I'm just trying to understand if something is implied or not. I can never tell and that's just how she and many other people phrase things.
Sorry for this random comment. I just saw your comment and had to get it off my chest. IDK what I am but I'm the worst at understanding implied things and I fall for jokes all the time.