Because it's normal for governments to run deficits. The truth is, a country like the UK can't run out of money and can always create more, not that that would always be advisable of course.
The fear of bankruptcy, especially for countries like the UK, is overblown because sovereign debt works differently from normal debt. We aren't borrowing from some strict creditor; we're essentially borrowing from ourselves and from the global market, which is largely driven by trust in long-term economic stability. The whole thing sounds like a giant house of cards when you think on it and it's quite scary to think just how little actually underpins our economy.
There's also the Modern Monetary Theory that states that taxes don't actually fund spending, rather government spending creates the need for taxes to control inflation and demand, particularly in countries with their own sovereign currency. Which would make a bit more sense, in my mind anyway, since government budgets never seem to be based on tax revenue.