S.ta.l.k.e.r is a fantastic game but not an rpg.
People always think that a "Role Play Game" has to have a skill tree or levels, or a character creator...but that is far from the truth. Having skill trees and levels are mechanics, the actual Role is what you do in the world with the stuff that is given to you, but in most cases the "freedom" given is not real, you are always following a path and in your journey you are aquiring new skills or abilities (or changing your look...wow that is so RPG!!).
As I said, you might have skills or abilities that makes you feel that that character is yours (not you but yours) but the main story never changes, your path is the one given and that is where it ends.
In S.T.A.L.K.E.R you don't really need any skill tree or levels, you just need to be you (or the character given but with your thinking) and do your own thing in the world with a minor path given at the beginning, then you may encounter things in the world but you are never forced to interact with that, you are always thinking of what to do next by your own. (and of course you have quests and such that might "guide you" but those are given like in most RPG's by talking with NPC's at certain places and so on, and there is something guiding you to an end as well, but that end changes because of your actions)
So to me it is a RPG, you can be a Stalker or join a faction if you so desire and make friends or enemies, ne neutral to them all (aside from those that hate everyone), and so on. You are the one that changes the path of the story like in almost any other RPG but even with more sense than in most of them to be honest.
If it is not...heck I never felt so much involved with my character in any game and felt that anything that I made in it was relevant to how it all eneded, not even with Geralt in The Witcher 3 felt that in the same level. Weird isn't it?