Because that's what defines a debt: a liability. The promised execution of your part of a contract.
I'm affraid *you* have an erroneous grasp on the concept of money. But again, read Rothbard.
Money is not "an honest man's promise", although this is a very common misconception.
Money is a valued asset, that is mainly used as an intermediate asset to store the value of the two halves of an exchange: the first half where you did something for someone else and obtained it, and the second half where you hope that others are going to do something for you.
It is easy to see where the "debt" concept comes from: you think that when you "do something for someone, society OWES YOU a return".
There could be a giant ledger, where all someone does for someone else, is valued, noted, and is a "credit" on the name of that person, which can afterwards make a claim on his contribution to society to "get something back".
However, that collectivist/communist vision is NOT what money is about. Money is not the big ledger of what you've done for others, and what you're entitled to to get back.
Money provides a similar macro-effect, but doesn't WORK that way. Money is rather based upon a free society, and not a collectivist-communist "organiser of all actions", which means that at every single interaction, you have to cover your valued actions for others against stuff others may want. Because if you don't, nobody's going to care about what was the good you did to society.
Money is a means to not to have to do "good for free". It implements the function to claim back your contribution, but not with an enforcible debt of society towards you (which is not possible in a free society), but rather with your owning of a desired and rare asset. The desire of others to possess that asset is what replaces the "debt of society towards you" but the responsibility to have it have value to make that claim, is entirely your speculative business.
Indeed, if money were to be a ledger of what good you did to society, then it would be strictly forbidden to *produce* money because that would give you the right on a claim without the beneficial action to society. It would be cheating in the ledger. This is what seigniorage is about. For money to implement "just return" it has to be a pure collectable.
And now you see that "digging up gold" would violate the "ledger of the debt of society towards you for former good deeds". And that is what it does, btw.
So no, gold is not a debt. But a rare asset of which not much can be produced, has similar effects, when accepted as money, as your debt-driven "ledger of all good deeds".