The thing that often happens is that people move on to other interests.  Once interests in Poser wane, they don't feel like taking the time to set up an account on another site and populating that site with everything they made.

This is something that freebie creators need to keep in mind when they set up their readme files. If they make their readmes less restrictive, their work can be shared amongst users forever. Otherwise, if they lose interest in Poser and let their website die, their creations become dead and unable to be shared to new users for about a century due to copyright laws. Some content creators may prefer having limited distributions on their freebies, but I'm sure that many would like to see them available for free forever.

My visual indexes of Poser content are at http://www.sharecg.com/pf/rgagnon