Random Access Memories

Sometimes I wonder why software systems constantly require users to change their passwords.

I suppose they assume we all have an infinite capacity to remember a string of numbers, digits and symbols are repeat that information anytime in the future without ever writing it down anywhere.

But you see, we often do. Most people have written down their passwords until they have memorized them.

I feel this is horribly insecure compared to allowing users to keep their existing passwords until they desire to change them.