You did the math without taking into an important factor. Networth ≠ Liquid cash. You see their networth is coming from stocks, bonds, investment on many things and their valuation, gold and simply any asset. So if you see someone valued at 10 billion that doesn't mean they can free up 10 billion and just buy bitcoin with it.

And if they try to liquid some of their asset like their company share of 10% that just crash their total valuation.