I lost the keys to my car for a few weeks. Thankfully, I had a backup set, but I literally lost the keys to my car.

For one, average human memory is not as ideal as you seem to believe, and for a specific counterexample, I'm smart, but my memory sucks. Oh yeah, sure, ask me to memorize a string of numbers, that I can do. But if you want me to remember to call the doctor, or to pick up something essential at the store, or where I put that electronic doodad (or my keys), my mind's a sieve.