Someone needs to maintain the wealth, keep avoiding taxes, keep an eye on regulations, foreign markets, looking for new investments.

Couldn't they pay someone to do that?

Yes. But this is VERY risky.

See this as an example: https://en.wikipedia.org/wiki/Madeleine_Schickedanz The article does not give much details. But managers basically stole her wealth from her and siphoned it to themselves.

"She was supported in her claim by Thomas Middelhoff". In my opinion it was Middelhoff who badly advised her for his own gain.

Money management is a huge industry.