I think writers write series for the simple reason that readers read (and thus demand) series.
And I think readers demand series simply because most humans are wired up that way (with some exceptions that maybe become literature professors or critics).
We are wired up to like a story and we like reading about new adventures and yet it seems there is also a need for familiarity to exist and persist.
Series give us that. Familiar characters, themes and milieu allow us to comfortably embrace new adventures.
I don't think this is a bad or good fact in itself, it is just the way we are.