DNA evidence is not perfect, all it tells you is that that person's DNA has been found there.
It doesn't tell you that someone was there, it doesn't tell you how it got there.
Barry George was convicted on a spec of gun residue, you could pick that up by sitting on a bus, or it could get there if the forensic lab are not as careful with their procedures.
Adding in to that the US system where your lawyer can sleep through the trial and not be considered ineffective.
As for, "What if it was your child / spouse/best friend..." what if they are guilty? They are still your child / spouse/best friend...