There are 3 reasons.

Because someone with his intellect and experience is probably already a multi-millionaire and cashing out opens up lots of ways to reveal his identity.
Because it is probably not 1 person, but a group which is already wealthy.
If he/they cash out and their identities are revealed, they will be prosecuted for tax evasion and all other "crimes" the government can conjure up, become world-famous and will never have a calm life again. Is having half a billion dollars worth being prosecuted and possibly having to spend 10 years in jail if you are already wealthy? Probably not.