Because unfortunately thats how mucked up society is, lack of self respect and people only wanting one thing. I do remember when I was young a few years back... I used to be the girl who wore jogging bottoms with crop tops/vests (not saying that is bad) but pushing my boobs up in my bra so they'd appear bigger, sticking my bum out in pictures and wearing tight thongs which to me was 'so cool' but unfortunately that didn't end up getting me anywhere. I hung out with the girls who were popular & slutty/beautiful which they were obviously aware of that (which I figured out in a few years) who got all the male attention, unfortunately they don't see that most of those guys only want them for one thing and see themselves as beautiful for it, it really didn't pay me off when I took advantage of this, most of the girls I know now that popped their boobs/bum out are now not s successful as the classy girls with self respect that I used to see as 'boring' but I really wish I was that way now, it didn't help me out, and so it will be for most of these girls and it will not pay off, they don't notice that. You want a guy who treats you with respect, not as a whore.
(Self experience and knowledge of people I knew). (Don't do it for attention girls, its not a good form of attention (sorry to disappoint you).