Government bonds can be popular parts of the investment mix of private pension funds because they pay out in the currency that those pension funds will need to pay out pensions. That removes any risk of exchange rate changes (say that you know you will have to pay out a billion euros in pensions in the next five years. A mix of technology shares statistically will do better in the long run, but in such a short timeframe, it could do worse, especially if it pays out in US dollar, which may go up or down relative to the euro)

Also, for governments there is a fairly easy way out of last resort: print money. The resulting inflation effectively gives everybody a cut on the government debts they hold, without the government having to negotiate a deal on that cut (that only holds if countries write debts in a currency they control. Debts in euros are somewhat of an exception in this respect, as are the rare country bonds in foreign currencies)