Super sexy friends as in girls or guys? If girls, I guess the guys do it for social proof. A lot of girls find guys attractive if they notice such guys are surrounded by attractive girls. If guys then the guys do it cause they don't know their friends are sexy and just post pics with their guy friends showing they have a decent social life and are not a bore.
What are they supposed to do? You're insecure and it is showing. This shouldn't matter at all. Someone appearing attractive makes you feel bad about yourself. I think you should work on your insecurities. Why do you feel so bad about yourself? Why should a man that seems interesting, having attractive friends matter?
I don't, but I've seen guys do that and it's fcking stupid. They think girls will think because he has girls with him he must be a good person or be seen as more desirable, but most girls will think of them as competition, or him as a player.