I think when a thing becomes enough of its own thing it can be considered not that other thing.
So although a hamburger meets many definitions of a sandwich, it is its own thing, and therefore not a sandwich.
If you said, "Hey, we're having sandwiches brought in for lunch today" and what is brought in is in fact a bunch of hamburgers, that would be surprising and confusing.
If you said, "Hey we're going out for lunch today, we'll go grab some sandwiches" then I'd assume you're going to hit a deli or Subway or Jersey Mikes, I would definitely not assume you were going to Five Guys or McDonald's for burgers.