Because there aren't enough pictures of tail-less cats out there to train on.
It's literally impossible for it to give you a cat with no tail because it can't find enough to copy and ends up regurgitating cats with tails.
Same for a glass of water spilling over, it can't show you an overfilled glass of water because there aren't enough pictures available for it to copy.
This is why telling a chatbot to generate a picture for you will never be a real replacement for an artist who can draw what you ask them to.
Oh look
It's one of those AI generated articles on a microblog pretending to be a news source that looks just like one of the same fake websites the ADF was using to push pro Trump propaganda last year
Oh wow that's not blatant at all