To further one of your points, there is very much pressure to keep closure rates up by pinning it on the most-plausible suspect even if you have doubts, and letting the DA sort it out. The DA does not always sort it out, and if it looks like an easy case then that someone gets railroaded.

Obviously this is not ideal at the police level (who should find the truth, not someone to blame) but some more accountability is warranted within the judicial system too. All incentives here are perverse, yet lack of them results in fuckall getting done.