I think it is measured and professionals who work in areas where this was relevant probably track this number among many others. I think the real question you are asking is why it is always framed as a % of GDP in public discussion and news reporting. Some people here point out how % of GDP can be more useful for comparing across countries with vastly different sized economies but I think we all know the real answer has more to do with how people perceive the value differently by using the larger number. Framing government debt as a % of GDP serves the zeitgeist, whatever that is.