Because when ever I ask this question, it turns out that the person has watched some really bad anime. I think it's a common sense question for anyone who hates an entire genre.
But that implies that they have to make the effort to watch a range of anime before deciding its not for them or that just their opinion of the idea of it isn't enough to make a judgement on. Like I said people wouldn't ask what porn people have seen before allowing them to decide that porn sucks so what makes anime different?
QFT
Or looking at it another way - I loathe opera. I have a strong aversion to the elements I associate with opera; interminably long scenes involving warbling in languages I do not speak, shrieking and wailing, melodrama. I have seen/heard very few operas, none for longer than half an hour (at which point the 'what the FUCK is this bollocks?' switch became operational). I have no intention of exposing myself to a wider range of opera in order to check that it is
all shit. So with anime. With genres or styles which are very distinctive it is not necessary to assume that those who hate it just haven't seen any good anime/porn/morris dancing/etc.
I'm aware there are probably one or two anime series/films which might be enjoyable if I can get over the initial distaste, but I can live without the discovery process personally.