Same as the dresses, apparently. The designers want their names in the news, so the movie babes wear the designers' latest frocks, prance around on the red carpet, and the designers hope to get some new clients with the exposure. Too bad so many of the dresses are awful!
Long, long ago, designers made "deals" with Queens, then with socialites, and later on stage actresses. People tried to copy the styles, as best they could and later, bought knockoffs. The Empress Eugenie made a certain style of hat famous, the then Prince of Wales ( Queen Elizabeth II uncle ) influenced what men wore, all over the world, and when people couldn't afford to buy the same kind of clothes of hats or jewels, they copied hair styles.
But yes, what you said about today is true; especially how absolutely UGLY so many of the clothes are!