If the fashions they design are not going to look good on their real female customers, then what’s the point?
Most fashion modeling is not intended to make sales to customers, but to wholesalers and retailers. They just want to see the garment.
Regarding sales to customers, my opinion is that about 90% of the people I see have no idea what looks good on them - it’s certainly not what they’re wearing - so their choices are driven by other factors.