When you have that much money it isn't possible to donate it all at once.

Fraud in charities is a real thing. There are a lot of "charities" that do some good work, but primary exist for the benefit for the benefit of someone. Often the primary purpose is to hide bribes: the CEO's spouse is a high government official. It is very easy to get mess up in such a charity and end up not doing well with your money.

The other problem is 90B is to much money for any charity to handle at once. Any charitable program that has a lasting (and thus useful) impact will take time. Even if the charity sets up a trust, there is nothing to stop the CEO from raiding that trust in latter years. Several charities started good, but over time have slowly - and legally - morphed into something that is very different from what the founders intended.

Staying in charge of his money is the best way to ensure that it is used well.