You don't buy from the designers directly, you buy from retailers, and they control the assortment.
They are also controlling the prices. When retailers haggle over a few bucks, manufactures skimp on fabric/trims/whatever they can to make the garment profitable. They stop grading things like length, so all sizes are the same length. They skimp on width, and they care more about fabric wastage. Especially now, when the cost to ship those goods has substantially increased.
The retail price is not set by the designer, the retailer places pressure on the designer/manufacturer to make something for a wholesale price they are willing to pay. Often the wholesaler/designer is agreeing to cover the loss in profits from a poor performing style for the retailer. The wholesalers are asked to help retailers make their margin by guaranteeing them.
Imagine that in order to sell your goods, the buyer tells you what it should cost, what it should look like, and that if it doesn't sell, they will need more money from you to cover their bad buy. That's fashion.
Designers also design for the average, even in the plus world. They need units to sell to the most amount of people, not the hard to fit people. There is no such thing as custom off the rack and there never will be--unless we start making goods to order and not in bulk. I can tell you right now, buyers are not buying for everyone. They have their "customer" profile in mind. You might not always be part of their target demographic.
And finally, retailers buy what sells. Designers have hardly any say at all in the process, aside from trying to convince buyers to buy something they believe in (which doesn't always work). For example, as a plus size woman, I can't tell you how many times I have been told what I want to wear by a non-plus size person in this industry. For so long I was told I wanted to be covered up with baggy, shapeless clothing. NO, I would say! Plus size (a term I personally dislike) wants to wear the same styles. They just want it to be more thoughtfully made for their body. That's it. The same stuff as everyone else, just made correctly for their shape.
A lot of manufacturers are still learning how to grade and make clothing for larger bodies. They don't understand how much fabric needs to be added as bodies increase in size. Bodies also increase in size less uniformly as they get larger. It's a more complicated formula. A great example is this: I recently bought a dress from Batsheva. It is a forgiving vintage styles dress. I thought it was a safe online buy. When it arrived, they over-graded the sleeve length. People grow in width more than in arm length. Just because I've gained weight in my mid-section, does not mean my arms got so fat they are longer. That's literally impossible, but they acted like an XL meant an XL sleeve length. At a certain point the ratio of grading length vs size needs to close or all-together stop. Arms aren't super long just because you are plus sized. This wasn't a poorly made dress and it wasn't cheap, but I am still going to have to alter it to make it look good.
Which brings me to my next point.
People who are saying clothing used to be structured and well-made are forgetting it used to also be more expensive and commonly altered. Most people do not tailor their clothes now, but you used to tailor everything. As a plus size woman, when I want to really look good, I get it tailored to fit. There is no other way.
I am a designer and a plus size woman. When I pay more for my garments, I tend to find that they fit better and are more thoughtfully designed (extra hidden snaps to prevent that bust level gap in a button-down shirt, for example). I understand the entire life cycle of every garment, of buying meetings, of retail floors, and of what it feels like to have spent a lifetime never truly seeing myself in fashion. I also know what happens when people won't pay more than what it costs to manufacture a garment--you end up with an industry built to supply poorly made goods to desperately make a profit from the sale. Everyone suffers.
My favorite line from my 17-year career: "They won't even pay the price of a sandwich." In reference to what a retailer would pay wholesale for a dress.
It's hard out here, and frankly, you get what you pay for. Sometimes, you don't even get that.
