Even good cops can, while being sincere in their quest for information, supply evidence that a good DA can use to convict an innocent person.
i.e. it’s not that the cops are necessarily “out to get you”. Rather, it is that the information, presented with the right twist, can land you in prison - even if you are completely innocent.
The side subject here is that you really, REALLY do not want to find yourself in a court of law as a defendant in a criminal case. At all costs.
Even if you are completely innocent of all wrongdoing, you can still easily find yourself being questioned by police.
Big example: domestic violence. You may be sure that your current girlfriend/wife is an angel who would never want harm to come to you. Are you willing to bet your life and future on her never getting pissed off enough at you that she claims you hit her or were "emotionally abusive"?
Second big example: one of your kids do drugs, and keep a stash in your house. You want to lose your house?