Could it be that the Zetsu and Obito becomming friends, but the Masked Zetsu will die eventualy so we dont see him.
As repsect for his old friend, Obito acting goofy and wearing a Mask with Zetsus face?
Same like kakashi coming late always to honor his "dead" friend, Obito.
They just have to be friends! :D
By the way, why these 2 Zetsus and Rin say "We are/I am watching you" it must heave a meaning.