Why has the public debt ratio risen so sharply as a trend in OECD countries and globally?

We are not looking at fiscal policy’s response to recessions and crises, but at long-term trends: why have public debt ratios risen so much since the 1980s? At first sight we can consider the following explanations: Public debt has increased in response to the fall in private debt (equivalently, the fiscal deficit has increased in response to the rise in private savings); this was true from 2010 to 2018; Public debt has increased because new public spending needs (pensions, healthcare, etc.) have appeared, for example due to population ageing and governments have not wanted (or been able to, due to tax competition) to finance this increased public spending by tax increases; this theory seems consistent with the facts; Public debt has increased because potential growth has slowed down, and it has been impossible to slow down public spending at the same time; this does not seem to be true.