I think it's significantly more reasonable to question why a stay at home parent uses full time daycare, rather than why a working parent.
A parent working is likely to be for specific positive reasons (paying bills, paying the mortgage, paying for a holiday, fulfilling ambition). Whereas a stay at home parent using full time care just ...does... not ...make... sense to the outsider. ('I want to be a stay at home parent... but im not looking after my kid 5 days per week'.... What the!). A stay at home parent using full time care is more likely due to some underlying negative reason (parent has mental issues etc).