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?
They were rich as millionaires, billionaires from something else rather than from Bitcoin, and they have been already rich enough to have no need of taking high risk. At least if they see Bitcoin as a risky thing even with good ROI over years since 2009, they would stay beyond this market.
Fortunately, you can see - I believe many people know about it too - a trend that more and more institutional investors joining this market. With Spot ETF approvals in January 2024, the last year, more institutional investors came and since then, there have been many days with not enough new mined bitcoins from new Bitcoin blocks to meet buying demand of Bitcoin Spot ETFs.