curry and durant can shoot. Also, curry has been injured a bunch as well. But both him and durant can create their own shots without needed to draw fouls or rely on athleticism and speed to blow past defenders.

Harden can shoot, but he relied so much on getting those foul calls that dried up. ANd his conditioning has gone out the window. He also had major injuries.
Westbrook was never the greatest shooter, and his game relied so much on jumping and running which he did so well, and still does pretty good. But very few athletic oriented point guards age well.

Also curry and durant have had it set up where they arent always relied on day in and day out to put their body on the line 100% of the time. Both harden and westbrook did just that. Westbrook gives his all every time he is on the court. He single handedly carried a terrible thunder team and a poor wizards team to the playoffs. That will take its toll (unless your name is lebron james apparently)