This is all in hindsight. When you are looking forward, with that kind of money, you have to certainly think about risk and diversify. That said, Bill Gates was more or less left with no options but to sell some MS shares. Almost all founder CEOs do this, 1 to de-risk their wealth and 2 to also perhaps to reduce exposure to scrutiny.

Whoever is sitting on 10B is not trying to 100x that money. They are trying to ensure that 10B doesn't go to 1M or worse to zero. The amount of risk you can take dramatically decreases with the increase in the amount of money you manage, which is why Berkshire is sitting on nearly $150B in cash and so is Apple. There are not that many less risky options to deploy 10s of billions let alone 100s.