I don't understand this. Even if they can't assess the risk in any one property, can't they estimate the total amount of flood damage in (say) the US each year, and spread the risk across thousands or millions of properties?

If the answer to that is that only people in areas vulnerable to flooding would buy insurance, and the companies would have to pay out too much - well, that means the insurance buyers are pricing the risks better, and the insurance company should hire some of them.