It comes directly from the math of double entry bookkeeping.
In my analogy, individual people do need to take care to ensure they don't run out of water. But you don't need to worry that if it rains a lot this year, there won't be any rain left for your children.
Right. It's not existential, sure. It helps that all our debt is issued in our currency. But the fact is that ~30% of our national debt is held by foreign entities, and at the very least interest payments are an outflow of wealth from our country. This is not a healthy position if we value freedom of action. It WILL eventually constrain our country in meaningful ways. At the very least the mechanisms to manage this debt will weaken our currency, leaving everyone that doesn't invest in the market behind (which is a sizeable portion of our citizenry).
Pointing out that earth is a closed system so it's all good doesn't address these very real concerns about our unchecked national debt.
In my analogy, individual people do need to take care to ensure they don't run out of water. But you don't need to worry that if it rains a lot this year, there won't be any rain left for your children.