Why are older women called cougars?

Because these older women are seen to be on the prowl for 'fresh meat', i.e. younger men.

Why are anime girls so attractive?

Animated women are something no real girl can ever really achieve to look like. And there is that feeling of wanting something that one can't get and wanting it more for that reason. They are perfectly proportional and always look beautiful not matter what time of day it is. That's a rare sight in the real world. Even really gorgeous girls have their off days. But an animated woman is always pretty, perfect, and ready for whatever happens to her while wearing a nice outfit. They rarely have flaws and in truth don't exude a feeling of a real girl. And otakus who are uncomfortable around a real, pretty girl can safely and without embarrassment, show their feelings to this anime girl without the fear of being ridiculed or rejected. While the beauty might not be real, no one can really stop from admiring something or someone that is beautiful, and no one can deny that anime girls are the perfect specimen.