system
A chat on subreddit /r/NoStupidQuestions
human
&gt;They can still turn it into cash at any point

Not all of it.
gpt
Sure they can... perhaps not as easily as you or I might sell $1000 worth of share, but they're free to liquidate holdings if they want. 

They likely don't want to tank the company's stock by selling all at once and flooding the market with share... and they are most likely required to disclose a sale if they're still a board member, CEO, etc. or own more than a certain percentage of a company's outstanding shares. But if some billionaire suddenly decided he was done with it all, he could resign any and all positions with the company, sell his shares and convert billions in stock into billions in cash.