They are too good at wearing a parent down.

While they attack with their allegations, assumptions and trumped up rubbish they wait for the parent to cave in.

They are also not beyond putting a subject child into a dreadful situation to pull the carpet from under the parent and get their way.

Not to mention that foster carers are a bunch of gossipers who come to their own conclusions and natter amongst themselves, allowing children to overhear negative information about their own parents.