Evem 99.999% is not good enough be enough because you will have at least one person that is guaranteed a false arrest and prosecution (which 90%+ prosecuted take a plea deal). There is inaccuracy with other methods as well but you have humans being held accountable. When it comes to justice,mistakes are tolerable so long as adequate compensatiom exists but when the mistake is systemic it becomes intolerable due to the preexisting guarantee of a mistake as opposed to a specific human making an error as a matter of chance. This is all exacerbated by other systemic cruelties of the US justice system where even an arrest and release for no cause means days if not weeks of imprisonment and if charged most people accept a plea bargain deal regardless of actual guilt. It's better to let actual guilty criminals get away than explicitly and systemically accept even one innocent person being punished incorrectly, because among other reasons, the justice system has legitimacy because its goal is to administer justice, accepting any amount of injustice invalidates that legitimacy and authority.

A good analogy would be a chef tolerating fecal matter in their food. Yes, there is always some small chance of that happening, but no one accepts food from s chef that explicitly treats toilet water and claims 99% of the fecal matter is gone and only one in 100 people will get sick from it.