Reason #1: His name. Magnus means "the greatest". There is no way that his parents named him that way expecting him to be good at something.
Reason #2: His calculating skill. No human being can solve a 10 move puzzle without knowing what the pieces are.
Reason #3: His appearance. He always looks like he just got out of bed.
Conclusion: Magnus Carlsen is actually Stockfish 16 Lite in a dead person's body.