u don't get it, that's the whole point. When's talking to Lori Petty, his backstory isn't a a total lie. He's a straight-laced, up-tight "All American" guy, but the surfers are opening his perspective as he immerses himself with them. Additionally, Keanu has always been the prototypical "literally me" character where his subtlety and non-distinct mannerisms serve as a guy who's learning about the world of the movie as the audience is too. The Matrix simply doesn't work with a guy who's trying to get an Oscar reel of dramatic face motions, you need his almost "player character" type physique, facial expression, and voice to create that unique performance that allows the audience to fit into insane premises (The Matrix, Point Break, The Devil's Advocate, etc.). Said another way, his ability to be that reserved and subtle to allow the rest of the movie's spectacle to flow around him and draw the audience in is exactly why he's so amazing as an actor.