Why do investors today think that OECD country government bonds are risk-free?

Investors today think that government bonds (issued by the United States, the United Kingdom, Germany, France, Spain, Italy and Japan) are risk-free or almost risk-free . Why? The usual answer is that governments can always hike taxes to ensure their solvency. But the tax burden is already very high in France, Italy and Germany, and it would be difficult to increase it in an environment of tax competition; Investors may think that central banks will always intervene to keep interest rates very low and monetise public debt in the event of problems, and stop governments from becoming insolvent and long-term interest rates from rising. They are probably right on this point.