It seems odd now, but up until the late 1960s, comics editors and publishers worked on the principle that their readership had a turnover every few years. With a few exceptions who were collectors, fans were thought to read comics for a few years around the age of ten and then move on to other interests. This is one reason why Golden and early Silver Age comics have such an irrational dreamlike quality to them... they were essentially fairy tales dressed up with modern trappings and did not have to make too much sense. Since very few fanatics errr fans kept collections of comics and followed them throughout their lives, it was thought safe to repeat yourself shamelessly. The kid at the newsstand or spinner rack had never seen comics from five or ten years back. So why not reuse some story ideas? This was particularly useful with this new Justice League which (since it was a reworking of the Justice Society) could easily go through the same paces its predecessor did. And there you have it. When getting ready to plot a new Justice League story, Julius Schwartz or Gardner Fox could look back over older DC comics there in the office and pick something that looked workable.
Of course, they also swiped shamelessly from the pulps (which had gone out of business around 1949 and which everyone thought were long forgotten). That Giant Turtle Man from JIMMY OLSEN was almost a tracing of a cover from THRILLING WONDER STORIES, for example. But that's another post.