Many of the covers are from the 1950s and the Mort Weiseinger era - the kind-of Limbo between the end of the Golden Age around 1947 and the kind-of start of the Silver around 1955.

I may be misremembering, but the cover-as-story-idea wasn't implemented until Julie Schwartz started his sci-fi revolution at DC in the late 1950s - that practice being common in the SF pulp magazine industry, which Julie had come from.

Me, I think they were just being goofy and trying to attract attention by creating covers that would get readers to buy the comic just to find out what the heck was going on.