Discussion of a finished series often continues for years, sometimes intensifying after the ending. What sustains it is the activity of theorising, which does not require the show to still be running.
Ambiguity is an invitation
Any long narrative leaves loose material behind: details introduced and dropped, inconsistencies from changing plans, and endings that deliberately withhold.
Audiences treat that material as a problem with a solution, because fiction is assumed to be designed even where it is actually the residue of production constraints.
Explaining an inconsistency is more satisfying than accepting it as an error, so the gaps become the most discussed parts of a work.
Theorising is a different activity from watching
Constructing an argument from evidence is participatory in a way that viewing is not, and it produces something the participant can show to others.
A good theory is a piece of work, with a claim and supporting detail, and it attracts responses that watching an episode alone never does.
That is why the practice continues without new material, since the pleasure is in the construction rather than in awaiting confirmation.
The community outlasts the subject
Groups formed around a series develop their own conventions, running jokes and reputations, and those relationships have value independent of the show.
Once new episodes stop, the group needs subjects to sustain itself, and rereading the existing material for overlooked detail supplies them.
This is why activity often continues at a steady level for years, dropping only when members drift toward other communities entirely.
Confirmation would end the game
A definitive answer from a creator resolves the question, and a resolved question generates no further discussion.
Communities frequently treat such statements as one interpretation among several, which preserves the ambiguity the discussion depends on.
Creators have learned this, and many now decline to settle questions about their own work, understanding that the uncertainty is doing something useful.
Creators write differently knowing this
Series are now often built with deliberate gaps and background detail intended to reward close attention and repeated viewing.
Done well this deepens a work, and done badly it produces the impression of significance without any underlying structure, which audiences detect eventually.
The difference tends to show at the ending, where a designed ambiguity holds up under examination and an improvised one does not.