Is Jeff worth all those billions because of how much of Amazon he owns? But no-one can buy all his shares, so is Jeff really worth anything at all? Like, I could invent a new coin and claim its value is £1m, but if no-one is allowed to buy it, I'm not really a millionaire.
Real wealth (goods and services) vs nominal wealth (money): as a society we've agreed that equity/shares in a business confer their market value on the holder. Most wealth is nominal, not real. The way to get ultra-rich is thus through equity.