This is somewhat of a sideshow, yet you may ask, "For what reason were there O-rings in any case?" Why would you be able to assemble the rockets in a single piece? The clarification I heard, from an old journalist sitting tight for a dispatch at Cape Canaveral, was the rockets were worked in Promotory, Utah, and transported via train to the get together structure. A passage among Utah and Florida had a bend in it, and that bend restricted the length of the rocket (or for this situation rocket portion) that could be moved.
There might be other structure choices that require a sectioned rocket, yet I speculate the political idea of NASA contracts, which has bits of significant projects worked in however many congressional locale as could be allowed, was a contributing variable.