Because the marathon runner trains for endurance And the sprinter focuses on his leg muscles to be faster but not be able to adapt unless he/she is used to the stress put on your body like the marathon runner has
You are doing all the work yourself in individual sports! If you are running the 100 meter dash, nobody is helping you run. It's all on you. Nobody will help you play 18 holes of golf, only you can. You need better physical endurance because you get no help with another person's endurance.
