Because Obito's MS doesn't cause blindness. Think about it; Obito doesn't have EMS, but he uses his power literally constantly. Clearly, there is something specific about his Sharingan that doesn't cause the usual blindness. I'm sure if Kakashi started using Amaterasu, he would start going blind.
Think about this.
Kakashi probably used Kamui close to 20 times.
Obito on the other hand has been using it constantly, Since the age of, we can assume 16 since that is around the time he fought the 4th Hokage.
2 Logical Explanations:
Obito doesn't lose eyesight because he has Hashirama Senju's DNA. Kakashi doesn't lose eyesight because he isn't Uchiha.
Or
The more obvious one, Kamui just doesn't cause eyesight to deteriorate.
Kishi couldn't have made this huge a plot hole. At least I don't think he would have made one that is so easily noticeable.
Obito and kakashi's eyes are linked! and Obito has Senju DNA inside himself now, which makes up for a lot of genome now.
please look at the following picture for reasons why i say they are linked.You must be registered for see images
I would argue that Obito's version of Kumai is the less powerful form, or at least takes less effort to activate than having to project it away from yourself. But even so, he virtually has a library of sharingans to borrow from granting his eye-sight starts to deteriorate, so it's plausible he has EMS.
I have a theory.
Kakashi was revived by Nagato. It is possible that his eye-sight got a "reset" thanks to that. Thus, he still has Kamui spamming left before he goes blind.
I have a theory.
Kakashi was revived by Nagato. It is possible that his eye-sight got a "reset" thanks to that. Thus, he still has Kamui spamming left before he goes blind.