This is not quite true. They don’t have a house. They rent a house at a different rate with different rules. For example should they be unable to pay their mortgage, the interest portion of which is pure rent, their house will be sold for them by their bank. This can occur if their mortgage is variable in an rising interest environment or if the economy tanks as a result of a housing crash, or because of the rising interest rates. So homeowners can and are affected by market crashes even if they don’t sell.