Look at it from the perspective of, as everybody likes to say, these days, "framing the debate."
If someone in 1937 told you to "create something we could call a superhero," there are few, if any, limitations, and you could have created literally anything, and if it was fun, it probably would have sold. Once Superman hits the stands, though, a superhero has a definition. To this day, the overwhelming majority of characters still fit the mold of the first dozen or so major superheroes. Look at the powers, the costumes, and even the structure of their names!
Yes, details get recycled, but that's like saying that some sitcom rips off the Honeymooners because they have a wacky neighbor that barges in. It's part of the lexicon. In the context of Captain America, most Victorian-era super-powered types are drug users. The shield shape (which MLJ did get Timely to change) is on the Seal of the United States. Comics were packed with kid sidekicks.
Maybe it was an intentional combination, maybe not, but I look at it like the odds of wearing the same outfit as someone else at the office or at a party. It's extremely low probability that it happens, but when it happens, it's more likely a coincidence than something creepy.
That's also why, for many big-name characters, there's a list of creators a mile long claiming to have played some part in the creation, I think. It's the infinite monkeys who wrote almost a quarter of Julius Caesar, and can't believe that someone wrote a whole play based on the historical character...
