Why do girl pose like this

They think it's sexy

It's girl o-clock

Time for dat booty

Because she has no booty, so she's sticking her back out to create an illusion. She went too far though

When "properly" arching the back, the chest and buttox protrude, the stomach is pulled back, the neck is elongated, and all "primary" and most secondary erogonous zones are featured. If you have a less than flat stomach, this pose helps hide it, and if you are not "well endowed" in either the butt or chest area those areas are given more prominence and the upper thigh appears as part of the glutes making them seem larger. It creates a maximum curvature, where traditionally a females curves are considered appealing. Finally it evokes imagery of several positions of intercourse.. if done correctly to keep a "line" down the body from head to toe and maintain visual balance. If not it looks awkward and to some either way it's lack of subtlety is less erotic than comical or offputing.

It's the thot clock

Try a make her butt look bigger cause she's got a chest like a new TV