" Why do vendors/builders not build more generic clothing, and let the end users adapt it to the figures of their choice in the apps they prefer to work in."

Most just want to apply the clothing and do their renders, not spend time bothering fitting and converting clothing (some enjoy doing that, most hate it) ,which should be as automated and as simple as possible in the first place. They wont buy something knowing that it wont really fit out of the box and requires some work from their side in order to work.

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