Ideally, if you paid off your mortgage by the time you reach your golden years, you have no/very little costs for shelter versus continuing to pay for rent. In your golden years, you still will need to pay for shelter somewhere otherwise…And in some cases your income in your golden years is much less than during your prime years.
Regarding the question “why not just save money?” It’s not so simple. A good portion of americans have proven they lack the discipline to save…A good portion are use to budgeting based on paying for bills…and for them, often times having more money on hand means more money to spend.