Updike has total ownership of his character in a way that fanfic writers do not, at the expense of a level of reader knowledge and involvement that fanfic writers can use. It is also, in my opinion, a major work of the Western literary tradition. I have met raku in physical reality and know hir real name and some of hir reasons for choosing to go the entity route, but they are not reasons ze is free to discuss in public. But it should not be overlooked that a number of entities do it because they like it. On this diagram I have color-coded the links between story files.
Green links involve no choices on the part of reader or character.
I believe that raku is exploring a philosophically different approach. It is a difficult and profoundly unsettling idea. Fan fiction gives raku a way to argue for or work through this disturbing idea using familiar characters with whom hir audience is deeply engaged. Much of the screen canon raku plays off and manipulates is from STV , which is unusual. Among other factors, writing about screen canon takes a lot of re-watching, and repeated viewing of STV can become actively painful.