I would prove you wrong but i'm not in my house where i have all the scans of when he's using them kamui spam in mangekyo sharingan and not 3 t/e.Well, here's your answer:
Kakashi only uses his Mangekyo in absolutely necessary situations, much like alive Itachi, while someone like Sasuke or Madara constantly spams it so they both lost their vision relatively quickly. Itachi took 8 years to finally start losing his vision, Kakashi has not even had his Mangekyo for a year.
People like you are the ones who scream plothole at everything they can not explain. Do you still think Tobi is Izuna or something?
Dam library.