The problem is that there are some attempts to make weaker test and try to confuse people so they think it's the real test. For example: https://en.wikipedia.org/wiki/DNA_profiling#Partial_matches
Quoting the Wikipedia article:
> We can see no reason why partial profile DNA evidence should not be admissible provided that the jury are made aware of its inherent limitations and are given a sufficient explanation to enable them to evaluate it.
That sound good, but every time I read it again it looks more and more fishy.
Say you can confirm a sample is 100% a certain person. How do investigators have any idea how it got there? How do you know it’s from a criminal and not the guy that stocked the convenience store shelves?
I tried to look into this and maybe I was searching the wrong thing but found nothing.
They often can't be certain, and I'm sure somebody is in prison because they're DNA was in an inconvenient place, but it's a far superior form of evidence than the other methods mentioned.
Unless a crime is widely watched in person with supporting recordings and DNA evidence, it's basically impossible to be a 100% certain of what happened. That's why court only attempts to prove guilt beyond reasonable doubt.
This is the same way we guarantee the integrity of elections.
Keeping a massive DNA database and using it to convict someone, anyone because a crime occurred near something they touched before would be fallacious and unjust. However, if we already had other reasons to suspect someone, and DNA gives us another, that's much more powerful.