Why do some think "relationship with god" and belief in god are same thing ?

You are right, believing in God does not mean you necessarily mean you have a personal relationship with God. To have a personal relationship with God , it means coming to terms with our sin, and accepting Christ as Savior, and then was we grow in our faith we do have a relationship with the Living God. Satan believes in God but that does not mean He is saved or has a personal relationship with Him. I have a personal relationship with God, however that is not exclusive to just a few. He has called everyone to repentance and to His family. He doesn't want anyone to miss the knowledge of the truth, that Christ came to earth to save us, and bring us into a relationship with the ONE TRUE LIVING GOD- not just believe in a God that is talked about in scripture. The whole point of the Bible is leading us into that relationship.