If you have $2bn worth of the same listed stock and go sell half of those, now you have a net worth of $1400m because your gargantuan order drained the order depth, tanked the stock value and triggered a panicked selloff at the stock market which further drove down that stock's price.

You can't take net worth away because it's just an estimate of what someone is worth. It may eventually be possible to turned into dollars and cents without losing too much in the process, but almost universally it can't immediately be exchanged in such a fashion.

Even more so when we're talking shares in a company that is not yet public, e.g. a founder's shares. At that point the valuation is complete speculation, based on what the company may be worth in some hypothetical future IPO. There's no actual price discovery since there's no public trading of such shares.

Net worth is usually not fully realizable unless it is in the form of cash. The larger the net worth, the smaller the realizable fraction usually is. In some cases, including some highly visible billionaires, the realizable fraction is likely tiny.