I am guaranteed to die at some point. Yet (at least in Germany) I can go to a private insurance company, agree to pay a few euros each month and in return they pay out 10000€ when I die to cover my funeral, no questions about cause of death asked. It's an insurance that is guaranteed to pay out, but it still insures against uncertainty because I don't know when I will die.
Flood insurance is fundamentally the same, the payouts are just not well spread, requiring a large geographically diverse insurance company.
Flood insurance is fundamentally the same, the payouts are just not well spread, requiring a large geographically diverse insurance company.