Taking aside all of the low self-esteem, bad relationships, possible mental abuse, etc - the normal reasons for a woman to not see herself as beautiful, you also have to understand... we literally see the worst parts of ourselves that no guy can really understand.
We are there for the gross moments (periods, cramping, without makeup, in grummy clothes, just woken up, when we are sick, etc), we see oursleves from the worst angles (looking down, through a phone, never in a candid moment, never with light in our eyes when we are speaking passionately, etc) and most importantly - most girls have a comparison issue. With this imaginary "her" that she can possibly one day be, but isn't at this moment. This girl in our head, the figure/person we want to me, more likely than not, it is unattainable. Which makes us see our flaws even more.
Not to mention the cast aside self-esteem issues, or the words that are etched in our skin from past criticisms or hate-filled comments. And when this woman is a little - all of that is magnified because... well... most littles are more sensitive (not all, I understand).
They don't say this to seek attention (well... not most of them) or to upset their CGs... but reality is... they don't see what you all are talking about. We don't see the girl you see. We can't see ourselves through your love-filled eyes, we see ourselves through our critical lenses. Sadly.