Who do you think makes these custom made clothes for them?
Rich people order custom made clothes from those very same luxury designer flagships in Yorkville. Every celebrity has a custom-made designer piece of attire. Be it for Met Gala, Oscars red carpet or some other event like that, it has to come from a well-known luxury designer brand and not some mom-and-pop sewing shop. Think of Kardashians/Jenners wearing custom Marc Jacobs and Schiaparelli to Met Gala, you can bet rich people and their kids will want the same sort of thing.
Hermes bags like Birkin are almost entirely customized for their clients for example. You have to have a purchase history with Hermes and get on the waiting list for such bag. Same goes with every other luxury designer and custom-made attire. There's no such service in Yorkdalle as far as I know. There are also boutique shops in Yorkville that carry inventory that simply do not exist anywhere else in the city. They sell some really rare stuff from Loro Piana, Brunello Cucinelli and Celine - real, old-money exclusives which are not that well-presented in Yorkdalle.