A paid-off house is a visible, reachable asset
Property ownership is a matter of public record and equity in it is not protected by a mortgage. For a household whose working years are behind them, that equity is not replaceable through future income, which makes a liability claim against it a different kind of event from the same claim against a thirty year old.
Retirement accounts are not the whole picture
Some retirement assets have protections and others do not, and the rules are specific rather than general. Planning on the assumption that everything is sheltered is how households discover the exceptions at the worst possible moment. An umbrella sits in front of the question entirely.
The cost is not proportional to the protection
A million dollars of additional liability cover generally costs a few hundred dollars a year, and the second million costs considerably less than the first. Measured against what it stands in front of, it is the least expensive thing on most households’ insurance.