Mostly it comes down to: division of labor is a thing.
CPS is responsible for things like foster care, finding if parents are (truly) neglectful and abusive, helping teach ignorant parents how to not kill their kids accidentally, and other related things. In most of the work they do, it is just due process - Our society has decided that when a parent goes to jail, or is ordered into an institution, or what not, and does so while caring for children and having no backup means for caring for those children, we shouldn't throw those kids out on the street and hope they survive.
In the case of neglect and abuse - it's apparent to people trained to find it (even in clear cut cases) but not as often noticable just by a quick glance: A common tactic of abusers is to hide it to the outside world, and scare the abused into lying about it - cops are trained to handle one situation, CPS investigators are trained to handle this different one. (Much like we have forensic accountants investigate financial crime and homicide detectives investigate murder).
When cops overstep and murder random people - the response is "well they have hard jobs, and they are trained to be safe", but when CPS oversteps and asks some uncomfortable questions, its "busybodies". Why does no one say "well they also get kids into homes rather than putting them on the street, and they see a lot of horrible abuse cases and are maybe a bit overcautious"?
Do you in general disagree with the notion of specialization and division of labor, or is it a knee-jerk reaction to the fact that "government" is doing it?
Why do you want to automatically shut down an agency that does a lot of good, because some bureaucrat is overstepping (or legally mandated to do so, whatever the case may be)?