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 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?
There is a saying "To each his own".
You can't force someone even if they're rich to buy bitcoin just because it's value appreciates fast. Each person has their own preference and bitcoin is very volatile and they are definitely aware that it is.
Those billionaires have enough investments and stream of income that they don't bother investing on a highly volatile asset.