Simply put, because you might not die on time. Average life expectancies work very well for insurance companies with thousands and thousands of customers who, on average, die on time. When one customer lives past their expected age, this is balanced –– from the insurance companies' point of view –– by another individual who unfortunately dies before their expected age.
But as an individual, you should not base your decisions on your life expectancy. You should plan for the possibilities of your own life, which include living longer than the average life expectancy.
Social Security is basically insurance against outliving your other resources and so, when deciding on your Social Security filing strategy, you should consider the possibility of living to your maximum age.
The default maximum age of 100 is a reasonable setting for most people, though others of course may want to adjust it depending on specific health issues. You can run reports with different maximum ages to explore the impact on your plans.