Because bitcoin's marketcap is worth trillions, not billions of dollars.
So you'd have to be a trillionaire, not merely a billionaire, to buy up most of the supply.

Secondly, you'd have to find enough people willing to sell you their bitcoin, and
since most are long-term holders it is simply not possible to find enough sellers to sell them to you
for you to stack most of the bitcoin in the world (over 50% of the total supply in existence).

Even if someone had the money, it is simply not possible to do it without massively driving up the price of bitcoin
in the process of attempting to accumulate more than 50 per cent of the supply. It would take years of accumulation,
not something that can happen over night. In the meant time you are competing with millions of others also trying
to buy a very scarce and limited asset, massively driving up the price. So it is simply not possible to do it.
.