TLDR: It’s partly climate change, but also in large part because regulations forbid insurers from raising rates on homes in disaster-prone areas. This creates perverse incentives for people to live where disaster will strike.
Something interesting about this situation is that it’s similar to the issues with the US health insurance system. You have a small minority of claimants who account for a large fraction of insurance claims, yet it seems politically unpalatable to allow insurers to jack up their premiums. Eventually, the government sets up insurers of last resort, so the public effectively subsidizes the riskiest policyholders.
One big difference is that whereas people struggling with health insurance are usually poor, the people facing issues with home insurance are often rich (e.g., those owning property on the Florida coast). And whereas it’s dubious whether the costs of getting sick have much deterrent effect, we do want a price signal to deter people from making their homes in disaster-prone places.