Because if you have access to that level of wealth and you want to keep quiet about it, you have access to the level of expertise needed in order to do a good job of keeping an extremely low profile. Bear in mind that once they reach a certain level, people with this kind of money go to great lengths simply to hide their true wealth. It's not a huge conceptual leap to accept than some will want to hide full stop.
As a kind of guide, try this out: grab the next copy of the Sunday Times rich list and see what you can find out about some of the genuinely rich folk you've never heard of. For every obnoxious attention-seeking Lewis Hamilton type, there's five billionaires who value the freedom that relative anonymity brings.