First of all, what they're doing is criminal and illegal and they know it, so it's natural they will want to hide it.  They also know that Child Protective Services can take away their children if they even appear to be abusing or neglecting them, and this can impact not only their personal freedom from jail and prosecution, but it impacts their income in the amount of monetary and physical aid they get from the government.

So they tend to resist or impeded investigations, and make promises to change behavior or environments that they have no intention of following through on.  Their only incentive is not to get caught or charged.