Same reason why athletes don't need a practical application for their exhaustive training. Advancing the theoretical frontier is a goal in itself, and it occasionally brings us to new scientific paradigms.
Chess players sit down and think very hard about how moving a rook now could affect the game five-six moves from now. No one actually cares about chess pieces outside of... well, chess players, and there's no real life constraint on moving only diagonally or only one square in any direction. Would you say chess has negative effects on one's life?