Whereas you get invited to update the program itself, unless you delay it, the individual items don't prompt you. Many people don't want or need the latest if they are happy with what they have. Compatibility is also an issue which might motivate some people to not update a specific item. For the same reason, when you go directly to the DLS, not from within CM, you can pick which version you want if more than one exists. As for the objects themselves, they get updated if and when the creator feels like it. Some people upload an item and then keep improving it as the state of the art advances. Others are happy with what they made or lose interest. Those items slowly get older as time passes. Of course, nothing prevents someone else from making another item that is an improvement by making a completely new original or by getting permission to update an existing object.