I always assume part of the job in those shops is to make those of us who spoil the view subtly unwelcome. And tbh I’m not even saying that critically - the entire vibe in designer shops is meant to be a tot
I agree, it's about curating the experience for other customers so that they are surrounded by as many similarly wealthy people as possible. It's the same in fine-dining restaurants. The waiters are obviously not paid bags more than waiters in other places but they will still be stern at guests who are breaking etiquette and spoiling the experience for other diners.
And it obviously doesn't boil down to looking or dressing a certain way. The shop assistants have worked in the store for long enough to instantly profile customers as soon as they come in the door. They're looking for body language, composure, language spoken, attitude towards asking for service or how they interact with the products.
Someone whose body language is just to "have a look" without the intent to buy is totally obvious. If you don't intend to buy but you are dressed beautifully and blend perfectly into the environment of the shop, then they will usually be polite and leave you alone. But if someone is clearly coming in for a browse and also dressed horribly then they need to shunt them along.
Another important factor is that luxury brands want customers to be billboards for the products. They favour those who look a certain way or will frequent certain locations so the products get seen by the right people. If a customer doesn't look visually or socially appealing, they don't want to sell you a bag even if you potentially have the money, because they don't want the product showcased on the outfit you're wearing in their store.