So let’s just get right into it. There are actually only a few reasons as to why Kakashi can’t turn off his Sharingan. And they are not very complex at all. Quite the opposite in fact.
It Was Transplanted
For whatever reason, transplanted Sharingan are not able to be activated and deactivated at will. Instead they stay activated at all times, which is why we see both Kakashi and Danzo keep their Sharingan eyes covered out of combat. This is an artificial way to turn the eye off and reserve the users chakra.
The same thing can be seen with Nagato’s Rinnegan, however he did not have to cover his up as the Rinnegan works in a totally different way then the Sharingan.
The Byakugan however is the exception to this rule, as Ao is able to turn his Byakugan on and off at will. This may be why the Hyuga are very protective of their eyes as they can be used by anyone pretty easily.
Kakashi Is Not An Uchiha.
It’s been shown that when an Uchiha takes another Uchihas eyes, they are in fact able to activate and deactivate their Sharingan. This is mostly shown through brothers taking their brothers eyes.
Example is Madara and Izuna’s eyes. (I’m aware that Madara didn’t forcefully take them this picture is just for example purposes).
And Sasuke taking Itachi’s eyes.
Kakashi however, is not an Uchiha (duh) thus he cannot turn off the eye gifted by Obito Uchiha.
So there you go guys. This was suggested by a fan and if you have any suggestions let me know and I’ll probably end up doing it. Peace out guys.