Most of the classic comic book characters were created under a "work for hire" arrangement where the creators assigned the rights to the publisher in exchange for their regular salary or page rate.
A few years ago, Lee sued Marvel, claiming that they had promised him a percentage of the Spiderman movie profits but then reneged. I'm not sure what happened to that suit. If other cases are any indication, the studio is probably trying to claim the film didn't turn a profit through creative accounting.
Additionally Jack Kirby and other ARTISTS who created the characters were the ones who renegotiated for ownership of their characters when the opportunity came up (I think 50+ years even work for hire can be renegotiated through changes in the copyright law).
Stan was a writer and editor but it doesn’t mean he came up with the characters.
Ahhh....the “creative accounting”....I understand that...
Poor guy.