Because the public is ignorant of the problems with it and the judiciary likes tools that can win easy convictions. Breathalyzers don't actually measure blood alcohol and there is no credible way they can estimate it to three significant digits but courts allow them too. Polygraphs are pure bullshit but the examiner's word that someone is "lying" is taken as sacrosanct evidence in court.

I don't know if this is the commenter's intent but isn't there a potential false positive issue with DNA testing?

Something along the lines of if only 0.01 percent of the world's population can match a DNA sequence found that still leaves us with a million matches where I just made up both those numbers.