Because they all look bloody gorgeous! 
I'm small in height and weight but managed to whilst living on cake for TEN months during my last pregnancy gain near on 4 stone 
Although I did lose the weight quite quickly afterwards, I felt so out of breath by the end of my pregnancy, and so big afterwards, that I don't want to gain so much this time.
So - is it safe to 'control' your weight during pregnancy - and in control I don't mean starve/cut intake below suggested calories ... I just mean eat very healthy foods and keep an eye on the cake which I will still eat at least once a week 
I have friends who throw away the scales for the duration of their pregnancies and eat everything in sight, and those who gain a neat bump, keep slim faces not like my balloon head! and slip back into their skinny jeans a couple of weeks after giving birth
I want to be in the skinny jeans group this time - is that so bad?
(p.s - I'm guessing a 4 stone weight gain is quite unhealthy really
)