Arthritic fingers don't aim well. Keys are somewhat self-centering and easier to operate without good accuracy.

Memory also degrades with age. Many people in their 80's and 90's start experiencing memory loss. My 85 year old father sometimes forgets the route to drive to the shopping center he has been driving to for decades. Relying on elderly people to remember a keycode or operate a smartphone is not a good strategy.

I suspect there is a certain amount of muscle memory involved in using keys. I don't even think "which key do I need" when locking/unlocking my doors, my hands automatically select the correct key.