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?
The answer to your question is actually quite complicated, but perhaps the reason is that not all billionaires believe in bitcoin, second most billionaires typically diversify their portfolios across multiple assets, furthermore most billionaires are leaders of public companies or large entities that must comply with strict financial regulations so they must be cautious if they want to invest significantly in bitcoin, however with the current adoption rate, we can clearly see that they are starting to believe in bitcoin as a way to generate profits.