Because the media and people are constantly telling them they aren't good enough.
We are taught that to be beautiful means having fewer zits/spots, having bigger boobs, having smoother skin, having a flatter stomach, having silkier hair, having a smaller nose, having bigger eyes, having longer lashes, having a smaller waist, having less hair, weighing much, much less etc etc.
No matter how big your boobs, smooth your skin, flat your stomach, silky your hair, small your nose, big your eyes, long your lashes, small your waist etc, you can always be better and are always told you should be better.
Being like that is not humanly possible (and isn't supposed to be), so often women feel very bad about the way they look and they feel ugly, even though they aren't. They spend their whole lives striving for an unachievable goal.
They belittle themselves so others compliment them, while they keep denying it, and the others tell them they pretty, they raise their moral
Yeah they just want attention
Not always, low self confidence
Because the makeup makes them feel over weight when they're not.
They don't really or the hsve body dysmorphic disorder