The Show Bible:
The show bible is created by the creator(s), and usually after the pilot episode has been written.
Staff vs. Freelance Writers:
It would be very unusual for the creator(s) to write every episode of a series, though it does happen. A staff of writers is hired to break stories, develop ongoing story lines, and write and rewrite episodes. Freelance writers (writers not on a show's staff) are not only often employed to write additional episodes, Writers Guild rules require freelancers to be used for a certain number of scripts per year.
The idea of writing all of the episodes before they begin shooting the first episode of that year is, to anyone who has worked in the trenches of episodic television, worthy of a true belly laugh. An average show is lucky to have a script ready for prep (the 7-8 day period before photography starts on an episode), and all too often the script is being written as it is shot.
Finally, there's plenty of room for talented, energetic writers who have the tenacity to fight their way through the fences surrounding the episodic business.