I feel I should also mention one more thing. The fact that you don't have the money until you sell the shares is also why "net worth" can be a highly misleading concept. (All numbers in the following example are fictional and made up on the spot, BTW). Billionaire Gill Bates, whose net worth is reported to be $20 billion, does not actually have 20 billion dollars. He has $5 million (million, not billion) of actual dollars in his bank account(s), but the rest of his net worth is in assets: he owns 200 million shares of MegaSoft Corp, whose share price is currently $100 per share. If MegaSoft Corp's shares suddenly drop in value (say, because a hacker group announces that MegaSoft's Doors 12 OS is full of, well, backdoors and suddenly nobody wants to buy it anymore) and now their shares are selling for $90, then Gill Bates's net worth will become 18 billion dollars instead of 20 billion. Did he "lose" 2 billion dollars in one day? NO. He never had those dollars. The "net worth" calculation is just the theoretical amount of money he could make if he sold all his shares.

And in fact, he could never actually make that amount of money by selling all his shares, because if he did put 200 million MegaSoft shares on the market, he'd never be able to find buyers for all of them at the current share price, and he'd be forced to drop his asking price by quite a bit before he managed to sell all 200 million shares. Not to mention the fact that if he tried to sell his entire holdings of MegaSoft Corp, many people would wonder what he knows about MegaSoft's long-term prospects, and would be afraid to buy those shares, driving the share price down even further. Gill Bates would be lucky to make $5 billion, let alone his theoretical net worth of $20 billion, if he were to suddenly sell all his shares. (If he sold them in a trickle over the course of ten years, he might well make the full $20 billion in the end, but not if he dumped them all on the market at once).

This is why (well, it's just one of the many reasons why) net worth is misleading. It's a theoretical number, but the actual amount of wealth someone has in practice entirely depends on market conditions at the moment they need the money, as well as how urgently they need it. (If the market is low right now, can they afford to wait six months for it to recover? Or do they need the money tomorrow and have to sell at a lower-than-ideal price?)