Because they seem to spend all day, every day posting on Mumsnet. They are never off the forum, popping up on every second thread, demonstrating an encyclopaedic knowledge of OPs' posting history, past threads, current threads etc etc
Who on earth gets these 'DC' out to school, and feeds them and clears up after them and supervises their homework and cooks their dinner? Do some of you have full time maids and nannies who do all that stuff while you lounge around eating chocolates and aibu-ing?