Bob Kane was an a**hole. Look how he stiffed Bill Finger out of co-creator recognition/royalties for decades over Batman.
I’ve even seen examples of autographs collected at cons back in the day where Kane always made sure his signature was significantly larger and more prominent than Kane’s after an agreement to sharing the creation was sorted out.
To be fair to the estates of Schuster and Siegel, wouldn’t you feel shafted if you’re creation (sold for $3,000 USD adjusted), when the corporates have made millions. Back then there was concept of syndication, streaming services, memorabilia, etc.
They were living hand to mouth at the time and had young family's to and need the money to make ends meet!
I’m sure if you created a property that went on to be come iconic and worths millions, you’d want your children and grandchildren to share in that success!
Look at the millions George Lucas amassed out of the merchandising rights to Star Wars because 20th Century Fox didn’t consider that side of the business to be profitable.