Because people believe it, and courts have accepted as fact that DNA evidence is infallible. Certainly there have been cases where DNA has been successfully challenged, but those are very rare. In the overwhelming number of cases where DNA evidence is present, it is a self-fulfilling prophecy for juries and judges.
If there is DNA evidence that is almost a guilty verdict. It should be more closely scrutinized but not everyone is rich enough to afford a real defense.
If there is DNA evidence that is almost a guilty verdict. It should be more closely scrutinized but not everyone is rich enough to afford a real defense.