> It covers a major individual cost and need (retirement, medical, housing) instead of just throwing it into a tax.

Forced saving makes it a tax. It's essentially no different than payroll taxes in the U.S. that fund Social Security. Buying government bonds is still marginally better accounting than a complete Ponzi scam like Social Security in the U.S., but even that ultimately amounts to the same thing - the government is paying itself, so it's a wash.

The social security trust fund does buy government bonds.