Not that they are not good ones, but they are not catch and shooters. Their strength is that they are creators on offense, and that's when they are most dangerous.
Nobody would turn Lebron, Jordan, or Kobe into catch and shooters, because they are a LOT more than that. Limiting their roles in to catch and shooters hurts your offense because they take away their strengths. It's like turning Nash into a catch and shooter. Can he do it? Sure, but that's limiting him.
Good catch and shooters go for much cheaper than Durant and Westbrook, tying up the majority of your cap on two catch and shooters will doom you.