It's not an RPG because you can not define any role in the game other than what is provided, and you are not given choices to define your path or your character, and and player skill trumps character skill in nearly every possible aspect.
Hence, the STALKER series are not RPGs. This isn't even debatable when it comes to this particular series.
Uh, have you even played STALKER? Its definitely not a full blown RPG, but there are quite a few different endings (6 I think?) depending on the choices that you make. It's got side quests, inventory management, upgrades, health system, and dialogue as well.