For me good adventure games are in large part character studies
If "Separation" from other Characters implies that the Player would usually be in those Characters' Company, then being separated from those is an important Part to any Character Study, namely the Difference between how a Character thinks, talks, behaves differently around People or when being alone, and possibly the Character suddenly finds out how helpless he or she is without those other Characters around.
If, however, you just mean the Player being alone for a long Amount of Time, then yes, you are right, Adventures are very much driven by their Characters, but what is a Character? Allow me to be a little broad here and make the Claim that an Environment is a Character, because it has Character. That is to say, the Environment tells a Story (or at Least it should), and thusly conveys a History, conveys Emotions, conveys Ideas, those Things that make a Character, and you also interact with that Environment.
Of Course, your Protagonist will still be making Comments about the Environment, I suppose, and thusly there still is Character Study via the Protagonist. (A nicely weird Overlapping of both Concepts is in "Edna & Harvey: The Breakout", where Edna, being insane, literally talks to the Furniture and Stuff and then starts little Mini-Dialogues... but I guess your Protagonist is a little too sane to do that.)
However, being alone in a Game where you usually aren't can also serve as a Way to evoke bad Emotions in the Player, Feelings of Anxiety and Helplessness, if done right -- but if done right in another Way, then the Player is not "alone", but in Solitude, having some comfortable Self-Time. (Which brings us back to Simon the Sorcerer, and I wonder if the Rocks having Faces helps making the Player feel less alone while roaming the Woods.)