Actually, we do not fully trust the police to handle evidence, which is why evidence bags with tamper-evident seals are supposed to be used, along with chain-of-custody records. There have been problems in the past with evidence tampering and sometimes it results in large numbers of cases being retried or verdicts being overturned because the invalidated evidence was so central to the prosecutor's case that a retrial would not be worth it. The problem here is that the "tampering" may not even involve the people handling the evidence, which allows a defense attorney to cast plenty of doubt on the evidence without having to challenge police procedures at all (after all, it could be that the evidence was corrupted by some random third party that has nothing to do with the case -- so why should the jury pay any attention to it?).