If you see someone in real life and don't see a flaw, they're highly likely to be good looking. There's no way in real life to hide weight with carefully chosen angles or bad skin with contrast tricks, etc. Our attractiveness detectors aren't built for photos.  
 
As far as why people don't learn from their mistakes; I think it's hard to look at a picture and think "well, she's probably 80% as good looking as she appears". We don't even know how to picture someone who is 80% as good looking as someone else (at least, I don't). 
 
Then there's the distortion of looking at a woman before you've slept with her. Women always look better before that.