Because it can be both. It's clearly a hard fucking job. That doesn't mean you dismiss claims of bias from the agencies own fucking employees.

I have a friend that's a single parent in a very high paying, demanding job. She makes a lot of money. She lives in a very high end apartment. One night her kid touched a hot pan while she was cooking. She rushed him to the ER - no serious injury though she felt horrible. The doctors told her they had to report the incident to CPS. She understood.

CPS shows up later that night, looks around in awe of the apartment and leaves within a couple of minutes. Now I know my friend isn't abusing her child. But that seemed to be a pretty cursory investigation for CPS to determine that. IF instead she was a single mother in a housing project, with plumbing that had not been fixed by the city and a lack of children's books - do you think that would have gone the same way?

In the end CPS needs to be judged on their consistency and transparency. They could absolutely be negligent in one instance, and biased in another. Those things aren't mutually exclusive.