Because he is a insufferable douche bag

The smile doesn't reach his eyes. When someone is genuinely smiling, the area around the eyes wrinkle up some (especially the outside corners). Fake smiles don't do that, not without conscious effort. It's so unsettling because it's setting off an instinctual urge of "this guy is being deceptive and is likely a threat". You can check by covering part of the face in the image: cover the mouth, and the eyes just look dead and soulless.

I'm not usually one to knock someone's appearance but you're right with this guy's weird friggin grin. Like it's not genuine or even appearing to be, in the slightest.

It's so cynical, cold, creepy and soulless. How did the A-B testing that drives his every single decision settle on that expression?