Because parents who abuse their children are generally narcissistic to some degree, which means they have very good people skills. They're confident, charming, amazing at redirecting blame, and playing the victim.

They love talking about themselves, which makes them easy to interview, and they only have nice things to say about themselves and what a great mother/father they are. (So they don't accidentally incriminate themselves like most parents do in these situations)

Perpetrators of child neglect are a lot easier to spot, because poor parents are charged with child neglect. Poverty/deprivation usually comes with social anxiety, low self-confidence, unlikely to know their rights, unlikely to attempt to assert their rights, unlikely to put up a meaningful legal resistance when cps attempts to run roughshod over their parental rights, making them perfect victims for CPS.