Quote: "Why would you want to do that? It's not like you can use it and release it to the public."
Because it's fun and not everyone wants to spent the time required custom modelling their game straight away.
It can still be educational using existing media from other games for personal experience. Not everybody is in a position to make the next best seller just yet and just want to experiment for a while first.
There seems to be a lot of people who shun the use of existing material in demos for no other reason than they have a moral stand point on it (not including you in that Ezykeyal).
Providing you don't try and sell or distribute a game with copyright material, no one other than a few grumpy forum members is going to care.