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?
Investment is about interest. Only a few billionaires believe that Bitcoin is a worthy investment. Others might not want to invest all they have in Bitcoin, so they end up diversifying their investment to different areas. A valid answer to this question would have come through research. Maybe OP can decide to investigate this question.