I’m not sure if I want to go into the full thing because people tend to get defensive about their preconceived notions and make a big, heated argument about it. But I will say this: game genres are defined by gameplay— not by content, by visuals, by storytelling style, or by similarities with other games people assume to be in that genre.
As simply as I can put it— and hopefully not opening up a huge can of worms— I define a role-playing game as a game in which your character(s) play one of several roles, meaning “classes”— each with their own stats and abilities that play differently and support the character(s) differently. You can have a single-character game where the character can choose one or more classes, or you can have multiple characters that each have their own classes, or you can have multiple characters that can choose between their classes. That makes D&D, Pokémon, Kingdom Hearts, Dark Souls, Final Fantasy XIII, and honestly a bunch of multiplayer shooters, etc., RPGs. That does not make Zelda or the first Dragon Quest/Dragon Warrior RPGs.
So the biggest problem with humans and categorizations is that humans are highly assumptive, seeing surface-level features and defining items by those, and defining items by outward similarities with other items that they already assume to be of that category. Because of this, what a lot of people do is confuse the adventure genre— games that use exploration, puzzle-solving, and key items in order to progress— and role-playing games, which almost always are adventure games as well. D&D? Both RPG and adventure. Final Fantasy XIII? RPG but not adventure. Zelda? Adventure but not RPG. But in most cases, RPGs are also adventures; so a lot of people through association mistakenly think games with common adventure elements are simply RPGs.
I know a lot of what I’m saying is going to fly over many peoples’ heads, and they’ll go crazy in the comments. Let’s see how long I can ignore them for the sake of my own sanity…