Because the fact that a person has dropped off the face of the earth without a trace is confirmation of a death.
Detectives can tell pretty easily if someone is dead or being held against their will. He's completely gone without an explanation or a reason: and in today's computer age if one isnt leaving electronic footsteps behind (ATM; Credit Cards, etc) they know one couldnt have engineered his own disappearance because he's not spending money to sustain himself.
So they've established he's dead.
Then the investigation starts from the inside out. Family to friends, to anyone who would have wanted to harm you. With forensics being what they are, you're gunna leave evidence behind ( microscopic dna naked to the eye can sink you); if you kidnapped a person, or killed them somewhere and transported the body, it cant be 100% clean. Plus with technology being what it is you cant hide from so many cameras and cell phone location tower pings that can place you either at the scene of the crime or away from an alibi you claimed. Mistakes, gossip...It all makes it almost impossible to commit a murder in this day an age.
...i used to watch the first 48 all the time and the homicide detectives on there didnt do anything but sit around and wait for someone to give them information...and even with someone telling them whodunit they still couldnt do anything unless the person confess...real detectives aint like columbo..goren and dci john luther...
