Why must an economy's income equal its expenditure?

A nation's economy is no different than your personal finances; You can't spend more than you earn.  Well, you can, but borrowing to live beyond your means is risky, even for a country.

While the Government can print as much money as it chooses, it has to have some kind of assets to back it up.  Otherwise, it will, eventually, not be worth the paper it's printed on.