Beacuse they died long time ago
Answer:
Through a circumstance known as "secondary transfer DNA", or "Touch DNA".
Explanation:
Most times when a crime is committed, DNA samples are obtained from surfaces in the scene where the crime was committed. There is a very huge possibility of picking up the DNA of someone who was never at the scene of the crime and this is a result of a condition known as Touch DNA.
Because we touch several objects which can be moved to different locations and touch people who are also always mobile, our DNA cells can find their ways to a crime scene where we had never physically been to. This can lead to false verdicts of guilt.
