There are billionaires worth over $100 billion, and even more worth between $10 billion and $100 billion. So why haven't any of them bought most of the bitcoins?
They do not want to put most of their money in bitcoin. If their net worth is up to billions, it doesn't mean that they have the billions in cash lol. It is probably in other assets too already and they don't want to convert all of that to buy bitcoin.
They could have done so, for example, in 2020, when the price of bitcoin was $5,000-$8,000. Bitcoin's market cap is now less than $2 trillion, and it used to be even less. Some multi-billionaire could have bought at least 10% of all bitcoins, but why hasn't anyone done so?
Because back then, they probably did not trust bitcoin to reach this point. If I knew bitcoin would be reaching $10k a decade ago, I would have done anything to buy some bitcoin even if I wasn't a billionaire myself.