Because there are no perfect detectives
There are no perfect crimes, but there are a lot of imperfect prosecutors.
One has to look no further than Mueller's doddering performance today, much to the Democrats' (and CNN's) dismay.
yes he seems very old and exhausted.
Because a perfect crime would mean leaving no trace behind.
Just because a criminal leaves DNA or fingerprints behind, doesn't mean the crime can be solved. Especially if the DNA/fingerprints are not in a database, somewhere.
According to something I read years ago ... the FBI claimed that the majority of criminals are caught because there were witnesses, or the criminal told someone.
I watch crime shows all the time, seems if you must do a crime wear odd sized shoes you will throw away right after and wear gloves. I can't even think of doing a crime, my hair is on everything I touch or get near.
A crime may go unsolved for many years, hopefully eventually most get solved.
If it was a perfect crime you wouldn’t even know they were dead/missing