Here's a thought that just occurred. If we were to base our idea of what public domain superteams existed just by looking at comic book covers, and not the stories within, we could easily find a dozen or more Golden Age superteams. Most publishers of anthology titles which featured several superhero characters seemed to have them all interacting on the covers, while inside the book, they all kept to their own individual adventures. It dawned on me that if you were say, looking at the covers of Timely's ALL WINNERS COMICS, it seems that they had a superteam almost from the beginning, yet in actuality, that team didn't come together until issue #19. The heroes of Harvey's SPEED COMICS interacted on covers, but not (with the exception of a single story) inside the comic... except on text page features that told "The Story Behind the Cover" (Captain Freedom, Shock Gibson, and Black Cat all worked together in quite a few team-up text stories). This is purely speculation, but my conclusion is that those covers existed mainly because it was a lot easier (and a better visual design, more eye-catching for the purposes of selling the comic) than trying to showcase the many different features in an anthology title in little separate images on the cover. Early anthologies tended to have the smaller, broken-up-by-panels cover design, or one main image bordered or surrounded by a bunch of character vignettes. One large, clean (but busy) image of several heroes interacting together just made better design sense from a selling perspective.
The "covers only" superteams also appeared on Fawcett's AMERICA'S GREATEST COMICS, Nedor/Standard/Better's AMERICA'S BEST COMICS, and Fox Features' BIG 3. Most of the larger comic book publishers had at least one title where superheroes interacted in cover scenes, and some had several. Even a few of the small-to-midsize publishers had a title or two, or at least an occasional "cover team-up" from time to time. There were smaller actual super team-ups in stories, too... like when PRIZE COMICS' page count shrank, they combined two of their longest-running superhero features, The Black Owl and Yank & Doodle, into a "Terrific Trio" of crimefighters, and that feature continued to run for a while. It might be fun to make lists of un-named superhero teams who existed only on covers, but not in the interior comics stories. You could find several by looking at nothing but the covers drawn by Alex Schomburg. I can only wonder what the general reaction of comic book readers was at the time. Did they ever write to the editors, asking "Well, why don't you put them all together in the stories, then?" The only company doing that on a regular basis was All-American Comics' ALL STAR Comics, with the Justice Society of America. Though of course that series is fondly recalled now as the first superteam, knowing how success breeds imitation among comic book publishers, you have to conclude that ALL STAR was not a blockbuster sales-topper... otherwise, ALL the other publishers would have been tossing diverse superhero features together as a team. Then again, maybe the JSA formula of interlinked, round-robin stories bookended by an intro and denouement where the whole gang got together was just too much work in plotting for most companies to want to bother?