Defendants are constitutionally entitled to a jury trial. Even if they were caught on 4k video, with their face fully identifiable, their fingerprints all over the blood scene, and they walk straight into a police station afterwards to confess.

A defense lawyer with a client who confessed on video has an extremely difficult job; they must try to present a credible case while also not suborning perjury or violating ethical rules. Very frequently, they're not going to have a "logical" case; they're simply trying to poke whatever holes they can in the prosecution's case...because the Defense is not constitutionally required to prove anything (unless they're attempting to prove an affirmative defense, and if they were you'd know because the judge would explicitly tell you).

On the other hand, if the prosecutor and the police and the forensics guys were all as incompetent as you claim, the defense might have gone to trial despite the taped confession because there was a chance they could prevail with a jury if the prosecution messed up presenting the case. I won most of my jury trials because the prosecution messed up.