You have been watching too many reruns of the "Forensic Files".
No physical evidence is needed, it's just easier to convict.
How the hell did murderers get convicted in 1980? Get real.
The notion that you need physicial evidence to convict someone has been called "CSI syndrome". Circumstantial evidence is just that - evidence. There are lots of criminals in prison today put there because of circumstantial evidence. You don't always have an eye witness, a smoking gun, a bloody knife, fingerprints and videotape. If you have a trail of circumstances that add up to overwhelming point to one person, then that person will be hearing the prison doors clang shut behind him.