When players like Magnus Carlsen and Viswanathan Anand finish their move, they sometimes look up while they wait. Shouldn't they be looking at the board to see their next move?
When you're that good at the game, you do not need to look at the board to have a complete understanding of the position. Especially just after your move is when you're more likely to be visualizing positional considerations and not directly calculating variations. So your thoughts are more general and do not depend on the specifics of the current position.
Moreover, looking at the pieces on the current board can be a distraction from the lines you are visualizing in your head. If a bishop is on d3 on the board in front of you, but you are calculating what happens when you play Bxh7+, you can sometimes get confused and falsely visualize having another bishop on d3 if you directly look at the board while calculating.
Many many people will look off into the distance or up in the sky when they are trying to recall something or to visualize something. At least at the start of a chess game, strong players are often trying to visualize and recall the exact move order of one or more "opening lines" that is favorable to their color. These are a very precise series of moves that must be executed in the exact order to assure arriving at a position where they will have an edge or where they have decided to introduce a novel move. So there is a lot of mental "making sure".
Also, there are usually monitors showing the position as a 2D diagram over each pair of players on the stage, and some players actually prefer that view to a 3D view.