Because There's So Much To Hate:

A source on the movie said, "Kate has a terrible self-image. She thinks she is fat and she is always complaining how certain outfits make her bottom look big. Of course, the reality is that she has the most amazing body. The script called for her character to be filmed nude in a shower, focusing in on her bottom and thighs. Kate insisted on a double being hired. She was quite open on the set about not liking her body and said she particularly loathed her bottom and was not comfortable baring it."

Can you believe that she hired a booty double for $2,000 a day? She's certifiably insane! Her a$$ is so freaking nice I would eat dessert off of it. I bet she's one of those chicks that knows she's hot and just does this stuff so her 'people' can compliment her on her flawless figure.