Because many creators are paranoid :matte-motes-angry:
Sure.. there are things you don't need to adjust. But when it comes to clothing then it's almost always: no mod = no sale for me because resizing never does it. I need more control because my avi has very likely not the same proportions as the creator's avi.
The argument that it looks bad if I adjust something myself is not valid: If I wear a skirt that goes half through my hips or if I can only resize it, it has a huge gap at the front or back then it looks like sh!t too. If I can adjust it myself, it will without exception look better than when I resize it only.
So creators who make clothing without mod-rights... just say: "I'm paranoid and I don't want you as customer, please go to someone that is less paranoid than me and buy what you want there". I do so anyway.