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The programmers didn't bother creating separate code for item menus and elevator menus.
Instead, they just created dummy items with floor names, and programmed the elevator menu as an item selection screen. The same is true about the Gym Badge man in Cerulean City, and this is the reason why the badge items also exist.
Elevator menus have special triggers that take the player to the proper floor after selecting an option, rather than trying to use the selected item. So, the floor items were never intended to be used, only displayed. If they are actually obtained and normally used, we get all sorts of wonderful glitchy effects.
This quirk in programming has some interesting consequences. For example, in Japanese games, you can actually swap options in elevator menus, just like items can be swapped in item screens. Obviously, this doesn't end very well: https://www.youtube.com/watch?v=fMf7_85ifc8
Also, activating the 'buy Master Balls for free' Gameshark code will cause the first option in elevator menus to be replaced with Master Ball. This bugged me out as a kid. The code is constructed to change the first item in any item list to Master Ball, and elevator menus are in fact item lists.