Something to be said for a superstar who somehow avoids major injuries. Whether it's training, natural physiology or luck, it matters.
We don't even need to discuss injuries to explain why the ultimate evolution of Manu Ginobili-Dwayne Wade has more standalone value than the ultimate evolution of Reggie Miller-JJ Redick. Being the third best player on a legendary team doesn't make you better than the top player on, say, a lottery team. It makes you great but you can't compare across categories, unless Klay goes to or gets traded to the Kings or Hornets or Pelicans or something and we can see him play alpha. Look at it this way - pick any 2 star players to start a team with. Would you rather have Harden or Klay as the #3 guy on your roster? If you want to play dumb player comparisons like this you have to understand their roles. You wouldn't compare Chris Paul and Jimmy Butler. Hakeem Olajuwon and Scottie Pippen. Trae Young and Giannis Antentokounmpo. They're not asked to do, nor do they want to do, the same things.
Harden is a man of God. No stress or inner demons from being a two faced snake all the time like the Warriors all stars. Stress can manifest itself into injuries.