FASHION SELLS. It sells magazines, people, movies, products, events, you name it. Fashion is big business.
The bigger the business, the more desirable it is for designers and international jewelry brands to participate. Both have been courting the red carpet crowd for decades. I see Dolce and Cabbana all over the stage in the music industry these day, hoping to capture wealth from the younger generation.
The co-dependence between celebrity and designer goes two ways. If you are a rising star, sporting designer labels and expensive jewelry only helps you get the eyeballs and following you need to climb further up your ladder. I buy your brand to get more eyeballs; you gift me fashion to exploit my eyeballs.
Because social media has put us all in charge of our own images and followings, and because I would personally prefer to have people 'like' or comment on a post than no response at all, fashion gives me the boost I need to stand out from the crowd. Take the hat, glasses, and matching bathing suit away in this picture, and replace it with a crew neck t-shirt. Most people would definitely scroll right past.
To me, posting on social media is the equivalent of going to an event or party. Posting your best self is a matter of self respect. Believe me, you are worth it!!