Keep in mind that Sharingan's are meant to be paired and not separate, and this is what creates kakashi's problem.
Kakashi's eye allows him to send distant objects to the Kamui realm.
Obito's eye allows him to send himself and objects he is touching to the Kamui realm.
Now that Kakashi's has found himself within the Kamui realm he is in the same situation as anyone else who would be there, stuck without a way to leave. yes, it is true that he has Kamui, but he cannot use it on himself. Him getting himself Kamui'd by Obito really was the ultimate sacrifice, as only Obito can let him out.
To further explain the eyes although it may not be necessary think of Kakashi's eye like a gun and Obito's like a grenade. The grenade blow up everything around it including itself but not distant things, the gun can shoot any and everything except for itself.