Lifespan of a typical lawn is not as long as you think.
Over-seeding lawn is what keeps it thick and fresh. A blade of grass lives an average of about 40 days. Your turf grass must continuously produce new blades (called tillers) to keep up with the ones that are dying back. When grass is started from seed, it starts off as a single blade. As it grows, dozens of tillers grow from the “crown” at the base of the plant. As tiller production exceeds the dieback rate, the thickness of the lawn increases. A healthy lawn depends on its ability to continuously produce more tillers.
The same thing is gong on with the roots. Roots die back just like the blades do. Over the course of a year, turf grass will shed it entire root system twice, replacing them with new ones. It doesn’t do it all at once, but a root at a time. The shed roots and blades help build up the organic matter in the soil.
Overseeding Lawn once per year is adding a new crop with new seed that is strong and ready to take over older plants that are dying or have died.