I can give you one reason the tent fit is so common. Take 100 women with the same height and BMI of 20. Will they all have exactly the same body proportions? Not exactly, but you could make a very tailored garment that would fit most of them. Take 100 women of the same height and BMI 40. Their body shape will vary a lot more. Body fat is stored in my different ways depending on the person and this gets amplified the higher in sizes you go. In other words some women carry most weight in the arms... so you cant do fitted sleeves, they have to be loose and flowy. Some carry in the midsection so you can't have a fitted waist with darts and such.. it has to be loose and flowy, etc. Making tailored plus size clothing can absolutely be done but you need to take an already small market and further restrict it to just one specific body shape, which most companies don't want to do. (The fashion nova body tends to be the exception here, hourglass with a big bust and bottom.) And perhaps there are more societal reasons for the tent look, but I'm not qualified to speak on those.