They knew they didn't own the characters. The revenue streams don't matter, the comic creators were working for the company and the company owned the things they made. If they didn't like it, they could have tried to go and build their own comic company, or just found a different industry to work in.
You're copyright discussion has nothing to do with the companies owning the characters we're talking about right now, with the exception of a few of the late 30s/early 40s characters who might have been public domain by now. Revenue streams only make sense when talking about things like movies and TV, which dealt with new types of royalties as things like streaming were invented. Comics didn't have royalties of any kind for decades, and even today they only (sometimes) have royalties from comic reprints, not from use of characters because the characters are owned by the companies. No creator who worked for DC/Marvel is owned a cent from things like movies unless there was some kind of contract or contract violation involved. I think it would be nice if Disney kicked back some money when using a character in a major role in a movie, but its as a nice thing to do, not a requirement.
A few quick points.
First, back in the late 30s/early 40s, there were lots of independent comic companies. However, as often happens, many were bought out or forced out of business by the companies that eventually because DC and Marvel. For a look at how truly ugly this sometimes got, please see the example of DC suing Fawcett (publisher of Captain Marvel, now Shazam) -
https://en.wikipedia.org/wiki/National_Comics_Publications,_Inc._v._Fawcett_Publications,_Inc.
Also, comics and other things like movies, syndicated TV, etc. are dependent on the distributor(s). While those are (mostly) unproven, the comic companies who survived often did so because of tight ties with distributors. For example, in 1937, DC was in debt to Harry Donenfeld, who was a pulp magazine publisher, printing-plant owner, and magazine distributor, so they ended up taking him on as a partner. Marvel's founder, Martin Goodman, also owned a newsstand-distribution company. I think they call that "vertical integration" now.
Second, revenue streams are somewhat relevant because nobody in those early days could have foreseen the movie, merchandising, and later TV and now digital money that these characters would bring in.
Third, you are incorrect about royalties. This has changed A LOT in the last few decades, thanks to organizing by creators since about the late 60s. When I saw Denny O'Neil at a con not long before his death in 2020, he said he'd gotten a check recently for an appearance by Maxie Zeus, a character he created for Batman comics in the 70s/80s. It all depends on your contract. George Perez was able to retire recently (health problems) because of the contracts he had with DC. The late comics writer Len Wein said he received more payments from Lucius Fox's appearance in the Batman films — he co-created the character — than he did for another co-creation, Wolverine, despite the success of the X-Men films (
https://smallbusiness.chron.com/much-comic-book-artist-make-per-project-12942.html). Obviously, the more popular the character and/or creator, the more leverage they have in their contract negotiations.