Being impossible to insure is related more to a high degree of synchronization of risk pools than overall danger of death. Death rates are averages - it is possible that one person dying per time period is rate-equivalent to the whole town dying at once in a once-in-a-tens-of-millenia event. But the risk exposure in that event is quite different.

You can get life insurance/death benefits as a mercenary or even as a centarian. It will be expensive because of the high risk of payout but you can get it. Because while it is probable that you will cack it from being so damn old/being a combatant in unstable regions with lesser support it is extremely unlikely every mercenary or centarian they have insured will die at once.