Yeah, while you are right that we dive into every video game, a roleplaying game focuses on the role of a character in the world. Tomb raider is not a focus of lara crofts in the world she is in. It is focused on controlling her with fun mechanics through puzzles. The focus is on plattforming and adventure. You can apply this to books and movies too. Sure you almost always have a main character but the genre is always different. The focus is the point.
It would be more difficult with SOMA. Soma is a fantastic mix of horror, adventure and rpg. I think what differenciates some more from normal RPGs is that in rpgs you slip into different roles of different characters in one game, and even play between them with yourself. For example in final fantasy 6 you play as many different roles, but it has a reason to do so. You are supposed to experience all those feelings between and of the different characters. So you have to play their role, if you want it or not.