When considering 'worth', you forgot to factor in the value of a constant income stream. In economic terms, a person is cash-flow positive if their income exceeds their outgoings. Even though someone may have an instantaneous snapshot of wealth that is equal to or less than zero, they may still be cash-flow positive or neutral.
Business valuations are related to total potential revenue which may be far less than the outgoings for a start-up company. However, that company may still be valued at millions or billions of dollars if the earnings potential is non-zero, even though they are hugely negative on their balance sheet. Think Amazon back in the early days when it first started.
This is the same for a person. If that person dies, they may have zero direct assets at the time (or may even owe more money than they have assets), but you'll quickly realise they were 'worth' a lot more than zero as all the debts and other things that were being paid for suddenly cease to covered. At that point, cash-flow takes a huge negative hit and debt is accumulating in a way that it can no longer be repaid. That debt may be in the form of dependants that now must be paid for by someone else (or by the State), unsecured loans that must now be written off by the creditors, and other general outgoings that will stop.
Say I somehow take out an unsecured loan for a million dollars from a bank. I am now worth $1,000,000 to that bank. If I die straight away, they are out the million. As long as I'm alive and working, I can keep repaying the loan, and at 5%pa over 10 years I'll end up repaying the loan, plus an extra couple of hundred grand in interest. So if you ask the bank what I'm worth, they'll tell you I'm worth a lot more than zero, even if I took the money to Vegas and blew it all in the high-rollers lounge.