Some people do it because their clothes are made that way and they never bothered to get new clothes. Some of my clothes show cleavage and I don't wear it to show off - just because I don't have a lot of options in clothes to wear. I have to buy new clothes soon.
Not to attract guys per se, but just to look feminine and cute. I have a boyfriend so not trying to attract guys, but that's no reason to change my style which does include a lot of tops that show cleavage.
Well in high school or maybe middle depending on your age it's definitely common to see girls showing cleavage and it might be to attract guys because that's an aspect of them that they know is made in a way that guys like
They do it for attention. They have no redeeming qualities or hobbies or anything interesting about them. They are vapid penis sheaths with mouths that are constantly talking yet without conveying a single intelligent thought.