How was Stan Lee not a billionaire?

Marvel was never financial solvent until relatively recently. They got bought out by Toy Biz around 1990 and they started getting their financial house in order. A number of properties were licensed out to studios to make make some money (Spider-Man to Sony, Hulk to Universal, Blade to New Line, X-Men to TCF, etc.).

All this to say, Lee probably got stiffed in this corporate re-shuffling by not getting lucrative deals to earn off the IP he had originated.

Lee was also swindled out of a chunk of change towards the end of his life.

Wikipedia:

Victim of elder abuse
In April 2018, The Hollywood Reporter published a report that claimed Lee was a victim of elder abuse; the report asserted that among others, Keya Morgan, Lee's business manager and a memorabilia collector, had been isolating Lee from his trusted friends and associates following his wife's death, to obtain access to Lee's wealth, estimated to amount to US$50 million.[34][35] In August 2018, Morgan was issued a restraining order to stay away from Lee, his daughter, and his associates for three years.[36] The Los Angeles Superior Court charged Morgan in May 2019 with five counts of abuse for events that had occurred in mid-2018.[37] The charges were false imprisonment, grand theft of an elder or dependent adult, fraud, forgery, and elder abuse.[38][39]

Another figure in the alleged abuse was Lee's former business manager Jerardo Olivarez, who was introduced to Lee by J.C. after his wife's death. Lee filed suit against Olivarez in April 2018, calling him one of several "unscrupulous businessmen, sycophants and opportunists" that approached him during this period. According to Lee's complaint, after gaining Lee's power of attorney, Olivarez fired Lee's personal banker, changed Lee's will, convinced him to allow transfers of millions of dollars from his accounts and used some of the funds to purchase a condominium.[40]

cuz he's dead

quote:

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I think it's debatable how much their heirs deserve for properties that weren't highly valuable until Disney created new value with Marvel Studios.

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Those close to Kirby have said that his main focus and worry was providing for his family. Lee claiming sole credit for Kirby's creations and refusing Kirby a writing credit were secondary concerns to Kirby not being fairly compensated. If there was ever a right that can be made for a wrong done to a deceased person, it's paying Kirby's kids and grandkids over and above what Kirby was owed.

I kinda assumed he did things like sell the rights for a Captain America movie back in the 70s for like $60,000.

You really want to talk about people who got the shaft - the comic book writers who wrote the stories that became the spine of massive Marvel movies. Some of them just got a few grand and tickets to a movie premiere.