Unfortunately your understanding, and that of nearly every juror in the US, is wrong. Tainted by the pro-police television narrative, that cops never mess up and that all crimes can be solved with the application of Forensic Science™. The link I provided elaborates further, but one reason from the concerningly long list: DNA testing really is little more than circumstantial evidence - it’s ridiculously easy for your DNA to show up at the scene of a crime you were not involved with. In general, it’s more precise than blood splatter or ballistic analyses, but not nearly by the margin you think.