Because the material thing, or totem, would be what determines reality. His head has created a scenario to assist him recover what he craves. He found the totem in oblivion. As our perceptions are soaked with our own theories and fantasies, determining absolute reality will forever be beyond our grasp.
It will be up to us whether we will give Cobb the reality that he always wanted, or we will forever lock him in this false reality called dream. It took Christopher Nolan 10 years to write that screenplay, and many will admit that they still glean a little more from it with each viewing.
Saito recognizes the carpet is not his, and calls Cobb out for keeping him within yet another layer of dream-existence.
Science fiction is the place for ideas, and this idea is a fascinating one.