There are plenty of people who only ride a couple of hundred miles in all those 20+ years. Also, it's not to difficult to ruin a leather saddle if you don't take proper care of it - and lots of people might not want to put any effort into saddle maintenance if they aren't very much into biking. Just throwing it into a dark corner and forgetting about it for a few years could make the saddle dry out and crack (or am I misinformed about this? I never actually stuffed away any leather saddles for years to observe the results, I must admit. :)) Subjecting it to excessive humidity may also be bad.
Who cares how much one rides when you have a saddle that can easily last 20+ years - they can be moved from bike to bike ya know. ;)