Why can't countries immediately stop the deficit? Governments often say things like "we will reduce the deficit by 2025".

Why can't they simply eliminate the deficit immediately by spending less than the taxes they receive?

Surely if a country is in debt, it should try and run a surplus so it can start paying off it's debts and reduce its interest payments.

Is there some reason why they can't do this?
Some countries have very strict fiscal rules that basically force them to keep on spending more, or reduce the government's ability to cut spending (Brazil is a good example). Other countries have looser rules, but can't cut because of political pressures.

Remember acute spending cuts mean less money for hospitals, schools and many other services that rely on public money. It's often a very impopular measure.

That's why economists often favor clear, well-defined spending rules. Such as establishing a primary surplus target during times of economic growth, a maximum deficit during recessions and maybe a debt/GDP ratio ceiling.