Why is it so hard to maintain a Product Backlog?

Number one problem I see with projects trying to implement Scrum is the lack of a Product Backlog.

This is strange because it’s one of the first things you have to do to start a project. I’s also one of the first thing you explain in Scrum : " You’re going to list features in a Product Backlog and these features will be developed and tested by small increments. "

How can the Backlog disappear so quickly ? Well it depends on the project. What is sure is that a lack of a Backlog make one thing visible and one thing invisible :

• It becomes visible that the Product Owner didn’t understand his role and job.
• It becomes invisible when the project is to end because no Product Backlog means no Product Burn Down.