Memories decay exponentially. This occurs both over time and over number of items to remember. Also, remembering requires brain attention. The vast majority of memories in a brain will never be activated sufficiently for conscious awareness. As memories accumulate, the fraction we actively access decreases.

The human mind is a flashlight that dimly illuminates the path behind. Moving forward, we lose sight of where we've been. Living a thousand years wouldn't make that flashlight any brighter.